Biology subjects

Geertsma, E. R.

Publications and source records attributed to Geertsma, E. R..

5 recordsLinked to original sources

bioRxiv2025

Absolute Quantification of Fluorescent Protein Fusions by Proteomics

Fusions with fluorescence proteins (FPs) play a pivotal role in experimental biology because of their sensitive and spatially precise visualization by spectroscopy. However, observed fluorescence is not always proportional to their molar abundance. Only a fraction of the fusion protein containing the mature fluorescence chromophore is detectable by spectroscopy and there is no generic method for estimating its molar abundance. We developed a fluorescence-independent mass-spectrometry based approach for accurate absolute sub-femtomole quantification of FP-fusions that also estimates the protein faction having fully matured chromophore. The method exploits an isotopically labelled 68 kDa recombinant protein standard containing peptide proxies for 6 prototypical FPs (mCherry, mScarlet-I, mKate2, EGFP, mNeonGreen and Dendra2) and two self-labelling (Halo- and SNAP-) tags. It enables quantification of proteins fused to any of more than 70 FPs or self-labelling tags. It is versatile, robust, precise and can be used broadly for the absolute quantification of fluorescent fusions in in-vivo and in-vitro cellular systems. As case study we combined mass spectrometry with fluorescence spectroscopy to study expression kinetics of FP fusions in cell-free systems. Molar concentrations of the expressed fusion protein, its fraction containing the mature fluorescent chromophore and of the fluorescent protein were integrated into a mathematical model to obtain kinetic rates of translation, chromophore maturation and folding.

biochemistry↗

bioRxiv2025

SLC26A11 is an atypical solute carrier with dual transport-channel function mediating lysosomal sulfate transport

Membrane transporters and channels are generally assumed to be based on distinct structural and functional principles. SLC26A11, a solute carrier with high expression levels in the brain, has been proposed to function as either an anion transporter or a channel. Here, we resolve this apparent discrepancy by demonstrating that SLC26A11 is a dual-function protein capable of operating as both a sulfate transporter and a chloride channel. By resolving its structure and combining biochemical studies and molecular dynamics simulations, we show that SLC26A11 exhibits all the hallmarks of a secondary transporter. The mechanistic basis for its selective ion transport identifies the protein as the elusive lysosomal sulfate exporter. Additionally, we demonstrate that SLC26A11 exhibits an uncoupled, channel-like chloride conductance gated by proton:sulfate symport. Our finding that the chloride-conducting state arises from the transport cycle may contribute to the development of novel therapeutic strategies for treating brain edema, and the identification of its role in lysosome sulfate efflux may provide new approaches to study and treat lysosomal storage diseases.

biochemistry↗

bioRxiv2025

GEQO biosensors for absolute analyte quantification in single cells

Genetically encoded fluorescent biosensors are widely used to monitor small molecule and ion levels in living cells. Quantitative FRET and FLIM sensors and highly sensitive intensiometric sensors have been developed for many analytes. Notwithstanding notable advances over the last years, a universal high-performance sensor design for absolute quantification that does not require specialized equipment has yet to be developed. We here report the GEQO platform of quantitative biosensors featuring calcium, ATP, cAMP, and organelle-specific variants. We used GEQO sensors to follow calcium and cAMP transients in immortalised cells, human pancreatic progenitor cells, and zebrafish embryos. We show that GEQO-based absolute quantification allows to account for analyte buffering and retains information in time trace data lost during relative quantification. GEQO biosensors will enable quantitative analyte measurements across a wide range of imaging platforms, a key prerequisite for diagnostic applications and quantitative approaches in basic cell biology.

cell biology↗

bioRxiv2021

FastCAT accelerates absolute quantification of proteins by using multiple short non-purified chimeric standards

Absolute (molar) quantification of clinically relevant proteins determines their reference values in liquid and solid biopsies. FastCAT (for Fast-track QconCAT) method employs multiple short (<50 kDa) stable-isotope labeled chimeric proteins (CPs) composed of concatenated quantotypic (Q-) peptides representing the quantified proteins. Each CP also comprises scrambled sequences of reference (R-) peptides that relate its abundance to a single protein standard (BSA). FastCAT not only alleviates the need in purifying CP or using SDS-PAGE, but also improves the accuracy, precision and dynamic range of the absolute quantification by grouping Q-peptides according to the expected abundance of target proteins. We benchmarked FastCAT against the reference method of MS Western and tested it in the direct molar quantification of neurological markers in the human cerebrospinal fluid at the low ng/mL level.

biochemistry↗

bioRxiv2017

Synthetic single domain antibodies for the conformational trapping of membrane proteins

Single domain antibodies called nanobodies are excellent affinity reagents for membrane proteins. However, their generation relies on immunizations, which is only amenable to robust proteins and impedes selections in the presence of non-covalent or toxic ligands. To overcome these key limitations, we developed a novel in vitro selection platform, which builds on synthetic nanobodies called sybodies. Inspired by the shape diversity of natural nanobodies, three sybody libraries exhibiting different randomized surface shapes were engineered for high thermal stability. Using ribosome display, exceptionally large libraries were pre-enriched against membrane protein targets and subsequently funneled into a robust phage display process, thereby reducing selection bias. We successfully generated conformation-selective, high affinity sybodies against the human glycine transporter GlyT1, the human equilibrative nucleotide transporter ENT1 and a bacterial ABC transporter. Our platform builds exclusively on commercially available reagents and enables non-specialized labs to generate conformation-specific binders against previously intractable protein targets.

bioengineering↗