Multi-Step Trafficking Pathway for Regulation of the Protein Composition of the Chlamydomonas Flagellar Membrane during Cilium-Generated Signaling
Hernandez Lara, Carmen Isabel
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The primary cilium is a signaling organelle found on most cell types in vertebrates. It is a distinct compartment for membrane and cytoplasmic proteins that mediate multiple sensory processes. Little is known about the cellular and molecular mechanisms that determine regulated ciliary localization of membrane or soluble proteins. Our laboratory uses the biflagellated green alga Chlamydomonas as a model system to study flagellar adhesion-induced signaling. During fertilization, Chlamydomonas mating type plus and mating type minus gametes undergo flagellar adhesion through the interaction of flagellar adhesion receptor activities (flagellar agglutinins) encoded by the SAG1 (plus gametes and SAD1 (minus gametes) genes, activating a cAMP-dependent signaling pathway that ultimately leads to cell fusion and zygote formation. Our lab recently developed a Chlamydomonas strain expressing a SAG1-HA transgene, making it possible to study directly the properties of the SAG1 gene product. We found that SAG1 is cleaved soon after its synthesis to generate a C-terminal 65kDa fragment (SAG1-HA-C65), which we showed is an integral membrane protein. In my studies here, I describe new findings on the properties of SAG1-HA-C65 and its trafficking into and from the flagellar membrane. I found that in resting gametes SAG1-HA-C65 was distributed over the entire surface of the cell body plasma membrane, with little in the flagellar membrane indicating that SAG1-HA-C65 was excluded from the flagellar membrane. After flagellar adhesion-induced gamete activation or after activation of gametes with di-butyryl cAMP, SAG1-HA-C65 rapidly became enriched in the flagellar membrane concomitant with becoming concentrated around the bases of the flagella at the apical end of the cell. Moreover, cytoplasmic microtubules were required for SAG1-HA-C65 apical accumulation and for its significant delivery to the flagella during signaling. Apical concentration and flagellar enrichment of SAG1-HA-C65 occurred in cells conditionally depleted of the anterograde intraflagellar transport motor protein, kinesin-2. Furthermore, studies on the fate of SAG1-HA-C65 showed that it was shed into the medium in a detergent soluble membrane fraction during flagellar adhesion. This work provides new insights into membrane protein trafficking that regulates the protein composition of the ciliary/flagellar membrane.