Worby, C. A. | bioRxiv Science

bioRxiv2021

Ca2+-dependent liquid-liquid phase separation underlies intracellular Ca2+ stores.

Endoplasmic/sarcoplasmic reticulum Ca2+ stores are essential to myriad cellular processes, however, the structure of these stores is largely unknown and existing models do not address all literature observations. We investigate CASQ1 - the major Ca2+ binding protein of skeletal muscle - and discover Ca2+-dependent liquid-liquid phase separation activity. The intrinsic disorder of CASQ1 underlies this activity and is regulated via phosphorylation by the secretory pathway kinase FAM20C. This divalent cation driven condensation demonstrates liquid-liquid phase separation occurs within the endoplasmic/sarcoplasmic reticulum, mechanistically explains efficient Ca2+ buffering and storage, and represents a largely unexplored mechanism of divalent-cation driven protein association.

biochemistry↗

bioRxiv2018

Ancestral Roles of the Fam20C Family of Secreted Protein Kinases Revealed by Functional Analysis in C. elegans

Fam20C is a secreted protein kinase mutated in Raine syndrome, a human skeletal disorder. In vertebrates, bone and enamel proteins are major Fam20C substrates. However, Fam20 kinases are conserved in invertebrates lacking bone and enamel, suggesting other ancestral functions. We show that FAMK-1, the C. elegans Fam20C ortholog, contributes to fertility, embryogenesis, and development. These functions are not fulfilled when FAMK-1 is retained in the early secretory pathway. During embryogenesis, FAMK-1 maintains inter-cellular partitions and prevents multinucleation; notably, temperature elevation or lowering cortical stiffness reduces requirement for FAMK-1 in this context. FAMK-1 is expressed in multiple adult tissues that undergo repeated mechanical strain, and selective expression in the spermatheca restores fertility. Informatic, biochemical and functional analysis implicate lectins as FAMK-1 substrates. These findings suggest that FAMK-1 phosphorylation of substrates, including lectins, in the late secretory pathway is important in embryonic and tissue contexts where cells are subjected to mechanical strain.

cell biology↗