Biology subjects

McKim, K. S.

Publications and source records attributed to McKim, K. S..

4 recordsLinked to original sources

bioRxiv2025

Exploring the C(2)M Cohesin Complex: Structure, Dynamics, and Ability to Facilitate Assembly of the Synaptonemal Complex

There are two meiotic cohesin pathways that regulate synaptonemal complex (SC) assembly in Drosophila. We previously proposed that C(2)M, which is required for SC assembly, is the only meiosis-specific component of a complex that includes Stromalin (SA), Nipped-B, SMC1 and SMC3. This model also predicts that specific residues within the C-terminus and N-terminus of C(2)M should interact with SMC1 and SMC3 to form a ring structure that may regulate the ability of C(2)M to facilitate SC assembly. Through mutant analysis, our results show several residues known to interact with SMC1 or SMC3 are critical for SC formation, suggesting that C(2)M may require a ring structure to perform meiosis-specific functions such as the formation of SC. We also show that SA colocalizes with and depends on C(2)M. However, the dynamics of C(2)M differ from SA and the SMCs in a way that suggests C(2)M regulates the dynamics and chromosome loading of the other cohesin proteins SMC1 and SA. Consistent with this conclusion, our results suggest that C(2)M can promote chromosome localization of the other cohesin components, and can induce SC assembly when ectopically expressed in germline mitotic cells.

genetics↗

bioRxiv2024

Distinct checkpoint and homolog biorientation pathways regulate meiosis I in Drosophila oocytes

Mitosis and meiosis have two mechanisms for regulating the accuracy of chromosome segregation: error correction and the spindle assembly checkpoint (SAC). We have investigated the function of several checkpoint proteins in meiosis I of Drosophila oocytes. Evidence of a SAC response by several of these proteins is found upon depolymerization of microtubules by colchicine. However, unattached kinetochores or errors in biorientation of homologous chromosomes does not induce a SAC response. Furthermore, the metaphase I arrest does not depend on SAC genes, suggesting the APC is inhibited even if the SAC is silenced. Two SAC proteins, ROD of the ROD-ZW10-Zwilch (RZZ) complex and MPS1, are also required for the biorientation of homologous chromosomes during meiosis I, suggesting an error correction function. Both proteins aid in preventing or correcting erroneous attachments and depend on SPC105R for localization to the kinetochore. We have defined a region of SPC105R, amino acids 123-473, that is required for ROD localization and biorientation of homologous chromosomes at meiosis I. Surprisingly, ROD removal, or "streaming", is independent of the dynein adaptor Spindly and is not linked to the stabilization of end-on attachments. Instead, meiotic RZZ streaming appears to depend on cell cycle stage and may be regulated independently of kinetochore attachment or biorientation status. We also show that dynein adaptor Spindly is also required for biorientation at meiosis I, and surprisingly, the direction of RZZ streaming. Author SummaryThe Spindle Assembly Checkpoint (SAC) is known to delay cell cycle progression until chromosomes are properly attached to microtubules. Meiotic cells often have modified cell cycle phases, and natural arrest points such as metaphase I in Drosophila. We show that in Drosophila oocytes, the SAC is sensitive to loss of microtubules, but not sensitive to a variety of kinetochore attachment errors. Thus, the function of the SAC appears to be limited to monitoring oocyte spindle assembly, and not required for accurate chromosome segregation. However, two of the SAC genes, rod and Mps1, are required for the biorientation of homologous chromosomes during meiosis I, suggesting an error correction function. Rod is part of the RZZ complex and is notable for its property of streaming off the kinetochores. However, our results show that streaming off the kinetochore may not contribute to RZZ regulation of microtubule attachments, and only be associated with SAC function. Instead, the establishment of stable end-on attachments may occur while RZZ is still present at kinetochore. We suggest that RZZ interacts with multiple motors to promote bidirectional movement of kinetochores along microtubules, which allows chromosomes to find and attach to the correct pole.

genetics↗

bioRxiv2023

A Dynamic population of prophase CENP-C is required for meiotic chromosome segregation

The centromere is an epigenetic mark that is a loading site for the kinetochore during meiosis and mitosis. This mark is characterized by the H3 variant CENP-A, known as CID in Drosophila. In Drosophila, CENP-C is critical for maintaining CID at the centromeres and directly recruits outer kinetochore proteins after nuclear envelope break down. It is not known, however, if these two functions require the same CENP-C molecules. Furthermore, in Drosophila and many other metazoan oocytes, centromere maintenance and kinetochore assembly are separated by an extended prophase. Consistent with studies in mammals, CID is relatively stable and does not need to be expressed during prophase to remain at high levels in metaphase I of meiosis. Expression of CID during prophase can even be deleterious, causing ectopic localization to non-centromeric chromatin, abnormal meiosis and sterility. In contrast to CID, maintaining high levels of CENP-C requires expression during prophase. Confirming the importance of this loading, we found CENP-C prophase loading is required for multiple meiotic functions. In early meiotic prophase, CENP-C loading is required for sister centromere cohesion and centromere clustering. In late meiotic prophase, CENP-C loading is required to recruit kinetochore proteins. CENP-C is one of the few proteins identified in which expression during prophase is required for meiotic chromosome segregation. An implication of these results is that the failure to maintain recruitment of CENP-C during the extended prophase in oocytes would result in chromosome segregation errors in oocytes. Author SummaryMeiosis in oocytes of diverse organisms, including humans and Drosophila, is characterized by a long prophase pause and a cell cycle arrest in meiosis I or meiosis II. These pauses could be a challenge for centromeres, whose components are replenished during G1, and then must remain with the chromosomes until the meiotic divisions. We have investigated the stability, prophase dynamics and function of centromere protein CENP-C. We show that CENP-C expression and loading onto centromeres during prophase is required for multiple meiotic functions. In contrast, the expression of other centromere partners CID/CENP-A and CAL1 is not required during meiotic prophase. Furthermore, expression of CID during prophase can be deleterious and result in ectopic kinetochore formation. CENP-C loading in prophase is required for sister centromere cohesion and kinetochore assembly. Our results provide the first description of CENP-C dynamics during meiosis and show that prophase expression is required for oocyte spindle assembly and function. CENP-C is among a small number of proteins that are required for the meiotic divisions but are loaded prior to prometaphase. Failure to maintain these proteins during the long prophase of oocyte meiosis may contribute to the increased aneuploidy associated with advanced maternal age.

genetics↗

bioRxiv2019

Cohesin and microtubule dependent mechanisms regulate sister centromere fusion during meiosis I

Sister centromere fusion is a process unique to meiosis that promotes co-orientation of the sister kinetochores, ensuring they attach to microtubules from the same pole. We have found that the kinetochore protein SPC105R/KNL1 and Protein Phosphatase 1 (PP1-87B) are required for this process. The analysis of these two proteins, however, has shown that two independent mechanisms maintain sister centromere fusion during meiosis I in Drosophila oocytes. Double depletion experiments demonstrated that the precocious separation of centromeres in Spc105R RNAi oocytes is Separase-dependent, suggesting cohesin proteins must be maintained at the core centromeres. In contrast, precocious sister centromere separation in Pp1-87B RNAi oocytes does not depend on Separase or Wapl. Further analysis with microtubule destabilizing drugs showed that PP1-87B maintains sister centromeres fusion by regulating microtubule dynamics. Additional double depletion experiments demonstrated that PP1-87B has this function by antagonizing Polo kinase and BubR1, two proteins known to promote kinetochore-microtubule (KT-MT) attachments. These results suggest that PP1-87B maintains sister centromere fusion by inhibiting stable KT-MT attachments. Surprisingly, we found that loss of C(3)G, the transverse element of the synaptonemal complex (SC), suppresses centromere separation in Pp1-87B RNAi oocytes. This is evidence for a functional role of centromeric SC in the meiotic divisions. We propose two mechanisms maintain co-orientation in Drosophila oocytes: one involves SPC105R to protect cohesins at sister centromeres and another involves PP1-87B to regulate spindle forces at end-on attachments.\n\nAuthor SummaryMeiosis involves two cell divisions. In the first division, pairs of homologous chromosomes segregate, in the second division, the sister chromatids segregate. These patterns of division are mediated by regulating microtubule attachments to the kinetochores and stepwise release of cohesion between the sister chromatids. During meiosis I, cohesion fusing sister centromeres must be intact so they attach to microtubules from the same pole. At the same time, arm cohesion must be released for anaphase I. Upon entry into meiosis II, the sister centromeres must separate to allow attachment to opposite poles, while cohesion surrounding the centromeres must remain intact until anaphase II. How these different populations of cohesion are regulated is not understood. We identified two genes required for maintaining sister centromere cohesion, and surprisingly found they define two distinct mechanisms. The first is a kinetochore protein that maintains sister centromere fusion by recruiting proteins that protect cohesion. The second is a phosphatase that antagonizes proteins that stabilize microtubule attachments. We propose that entry into meiosis II coincides with stabilization of microtubule attachments, resulting in the separation of sister centromeres without disrupting cohesion in other regions, facilitating attachment of sister chromatids to opposite poles.

genetics↗