Biology subjects

Snell, W. J.

Publications and source records attributed to Snell, W. J..

5 recordsLinked to original sources

bioRxiv2025

Molecular determinants of species-specific cell-cell recognition activating the class II gamete fusogen HAP2

Species-specific adhesion of complementary gametes is a prerequisite to cell fusion during fertilization. Despite its importance, the molecular mechanism underlying cell-cell recognition and membrane adhesion to drive gamete fusion is not fully understood for any organism. In the alga, Chlamydomonas, the species-specific gamete adhesion protein MAR1 guides the fusogen HAP2 into a pre-fusion conformation on mt- gametes and serves as a receptor for the FUS1 adhesion protein on mt+ gametes. Here, we show that soluble recombinant MAR1 completely blocks the HAP2-dependent gamete fusion process by binding to FUS1 on mt+ gametes and report the X-ray structure of the MAR1-FUS1 complex. In vivo gamete fusion experiments showed that mutations of key residues at the observed MAR1-FUS1 interface strongly impair adhesion, yet gamete fusion still proceeds, highlighting the efficiency of the fusion-activation process. These findings uncover the molecular architecture of a surface receptor complex whose formation releases the viral class II viral fusion-protein ortholog HAP2, allowing it to undergo its membrane-fusogenic conformational change.

cell biology↗

bioRxiv2025

The membrane topology problem inherent in ciliogenesis is solved by ciliary recruitment of a phospholipid flippase

The cilium is a slender cellular projection whose membrane transduces signals essential in development and homeostasis. Although assembly of the axonemal core of the organelle is well-studied, the mechanisms that underlie formation of the [~]250 nm diameter ciliary membrane from oppositely curved 50-100 nm diameter pre-ciliary vesicles remain poorly understood. Here we report that during ciliogenesis in the bi-ciliated alga, Chlamydomonas reinhardtii, aminophospholipid flippase 2 (ALA2) is recruited laterally from a reservoir on the plasma membrane onto the nascent ciliary membrane where it transfers conically shaped phosphatidyl ethanolamine (PE) from the outer to the inner leaflet. In cells lacking ALA2, initiation of cilia regeneration after experimentally induced de-ciliation is delayed, PE flipping is impaired, the ciliary membrane becomes distorted, and cilia that do form are shorter than wild type. Our results uncover a membrane topology problem inherent in ciliogenesis and demonstrate that it is resolved by a phospholipid flippase. HighlightsO_LIMembranes of pre-ciliary vesicles reverse curvature to become the ciliary membrane C_LIO_LIUpon initiation of ciliogenesis, plasma membrane ALA2 transits laterally to cilia C_LIO_LIThe ciliary membranes of ala2 mutants fail to conform to axonemal shape C_LIO_LIALA2 flips PE to establish ciliary membrane curvature C_LI

cell biology↗

bioRxiv2021

A cytoplasmic protein kinase in Chlamydomonas couples engagement of ciliary receptors to rapid cellular responses

The principal function of the primary cilium is to convert cues from the extracellular milieu into changes in cyclic nucleotide concentration and cytoplasmic responses, but fundamental questions remain about the mechanisms of transmission of cilium-to-cytoplasm signals. During fertilization in Chlamydomonas reinhardtii, ciliary adhesion between plus and minus gametes triggers an immediate [~]10-fold increase in cellular cAMP and activation for cell fusion. Here, we identify Gamete-Specific Protein Kinase (GSPK) as an essential link between cilary receptor engagement and gamete activation. The ciiary adhesion-induced increase in cAMP and cell fusion are severely impaired in gspk mutants but fusion is rescued by a cell-permeable form of cAMP, indicating that GSPK functions upstream of the cAMP increase. GSPK is cytoplasmic, and, remarkably, the entire cellular complement is phosphorylated in less than 60 seconds after ciliary contact. Thus, a cytoplasmic protein kinase rapidly converts a ciliary membrane cue into a global cellular response.

cell biology↗

bioRxiv2021

MAR1 links membrane adhesion to membrane merger during cell-cell fusion in Chlamydomonas

Union of two gametes to form a zygote is a defining event in the life of sexual eukaryotes, yet the mechanisms that underlie cell-cell fusion during fertilization remain poorly characterized. Here, in studies of fertilization in the green alga, Chlamydomonas, we report identification of a membrane protein on minus gametes, Minus Adhesion Receptor 1 (MAR1), that is essential for the membrane attachment with plus gametes that immediately precedes lipid bilayer merger. We show that MAR1 forms a receptor pair with previously identified receptor FUS1 on plus gametes, whose ectodomain architecture we find is identical to a sperm adhesion protein conserved throughout plant lineages. Strikingly, before fusion, MAR1 is biochemically and functionally associated with the ancient, evolutionarily conserved eukaryotic class II fusion protein HAP2 on minus gametes. Thus, the integral membrane protein MAR1 provides a molecular link between membrane adhesion and bilayer merger during fertilization in Chlamydomonas.

cell biology↗

bioRxiv2018

Fusion surface structure, function, and dynamics of gamete fusogen HAP2

HAP2 is a class II gamete fusogen in many eukaryotic kingdoms. A crystal structure of Chlamydomonas HAP2 shows a trimeric fusion state. Domains D1, D2.1 and D2.2 line the 3-fold axis; D3 and a stem pack against the outer surface. Surprisingly, hydrogen-deuterium exchange shows that surfaces of D1, D2.2 and D3 closest to the 3-fold axis are more dynamic than exposed surfaces. Three fusion helices in the fusion loop of each monomer expose hydrophobic residues at the trimer apex that are splayed from the 3-fold axis, leaving a solvent-filled cavity between the fusion loops in each monomer. At the base of the long fusion loop, Arg185 docks in a carbonyl cage. Comparisons to other structures, dynamics, and the greater effect on Chlamydomonas gamete fusion of mutation of axis-proximal than axis-distal fusion helices suggest that the apical portion of each monomer could tilt toward the 3-fold axis with merger of the fusion helices into a common fusion surface.

biochemistry↗