Chiang, W.-C. | Search bioRxiv

bioRxiv2023

Male germ cell-associated kinase (MAK) is required for axoneme formation during ciliogenesis in zebrafish photoreceptors

Vertebrate photoreceptors are highly specialized retinal neurons that have cilium-derived membrane organelles called outer segments (OS), which function as platforms for phototransduction. Male germ cell-associated kinase (MAK) is a cilium-associated serine/threonine kinase, and its genetic mutation causes photoreceptor degeneration in mice and retinitis pigmentosa in humans. However, the role of MAK in photoreceptors is not fully understood. Here, we report that zebrafish mak mutants show rapid photoreceptor degeneration during embryonic development. In mak mutants, both cone and rod photoreceptors completely lack OSs and undergo apoptosis. Interestingly, zebrafish mak mutants fail to generate axonemes during photoreceptor ciliogenesis, whereas basal bodies are specified. These data suggest that MAK contributes to axoneme development in zebrafish, in contrast to mouse Mak mutants, which have elongated photoreceptor axonemes. Furthermore, the kinase activity of MAK is critical in ciliary axoneme development and photoreceptor survival. Thus, MAK is required for ciliogenesis and OS formation in zebrafish photoreceptors to ensure intracellular protein transport and photoreceptor survival. Summary statementMale germ cell-associated kinase (MAK) is a cilium-associated serine/threonine kinase that promotes axoneme development during ciliogenesis in zebrafish photoreceptors to ensure intracellular protein transport and photoreceptor survival.

cell biology↗

bioRxiv2023

Side-Chain Dynamics of the α1B-Adrenergic Receptor determined by NMR via Methyl Relaxation

G protein-coupled receptors (GPCRs) are medically important membrane proteins that sample inactive, intermediate, and active conformational states characterized by relatively slow interconversions ([~]s- ms). On a faster timescale ([~]ps-ns), the conformational landscape of GPCRs is governed by the rapid dynamics of amino acid side chains. Such dynamics are essential for protein functions such as ligand recognition and allostery. Unfortunately, technical challenges have almost entirely precluded the study of side-chain dynamics for GPCRs. Here, we investigate the rapid side-chain dynamics of a thermostabilized 1B-adrenergic receptor (1B-AR) as probed by methyl relaxation. We determined order parameters for Ile, Leu, and Val methyl groups in the presence of inverse agonists that bind orthosterically (prazosin, tamsulosin) or allosterically (conopeptide {rho}-TIA). Despite the differences in the ligands, the receptors overall side-chain dynamics are very similar, including those of the apo form. However, {rho}-TIA increases the flexibility of Ile1764x56 and possibly of Ile2145x49, adjacent to Pro2155x50 of the highly conserved P5x50I3x40F6x44 motif crucial for receptor activation, suggesting differences in the mechanisms for orthosteric and allosteric receptor inactivation. Overall, increased Ile side-chain rigidity was found for residues closer to the center of the membrane bilayer, correlating with denser packing and lower protein surface exposure. In contrast to two microbial membrane proteins, in 1B-AR Leu exhibited higher flexibility than Ile side chains on average, correlating with the presence of Leu in less densely packed areas and with higher protein-surface exposure than Ile. Our findings demonstrate the feasibility of studying receptor-wide side-chain dynamics in GPCRs to gain functional insights.

biophysics↗