Ciliary protein turnover continues in the presence of inhibitors of golgi function: evidence for membrane protein pools and unconventional intracellular membrane dynamics

Newly synthesized polycystin-1 takes different trafficking pathways to the apical and ciliary membranes

Mutations in the genes encoding polycystin-1 (PC1) and polycystin 2 (PC2) cause autosomal dominant polycystic kidney disease. These transmembrane proteins colocalize in the primary cilia of renal epithelial cells, where they may participate in sensory processes. PC1 is also found in the apical membrane when expressed in cultured epithelial cells. PC1 undergoes autocatalytic cleavage, producing an extracellular N-terminal fragment that remains noncovalently attached to the transmembrane C-terminus. Exposing cells to alkaline solutions elutes the N-terminal fragment while the C-terminal fragment is retained in the cell membrane. Utilizing this observation, we developed a "strip-recovery" synchronization protocol to study PC1 trafficking in polarized LLC-PK1 renal epithelial cells. Following alkaline strip, a new cohort of PC1 repopulates the cilia within 30 minutes, while apical delivery of PC1 was not detectable until 3 hours. Brefeldin A (BFA) blocked apical PC1 delivery, while ciliary delivery of PC1 was BFA insensitive. Incubating cells at 20°C to block trafficking out of the trans-Golgi network also inhibits apical but not ciliary delivery. These results suggest that newly synthesized PC1 takes distinct pathways to the ciliary and apical membranes. Ciliary PC1 appears to by-pass BFA sensitive Golgi compartments, while apical delivery of PC1 traverses these compartments.

A role for the membrane in regulating Chlamydomonas flagellar length

Flagellar assembly requires coordination between the assembly of axonemal proteins and the assembly of the flagellar membrane and membrane proteins. Fully grown steady-state Chlamydomonas flagella release flagellar vesicles from their tips and failure to resupply membrane should affect flagellar length. To study vesicle release, plasma and flagellar membrane surface proteins were vectorially pulse-labeled and flagella and vesicles were analyzed for biotinylated proteins. Based on the quantity of biotinylated proteins in purified vesicles, steady-state flagella appeared to shed a minimum of 16% of their surface membrane per hour, equivalent to a complete flagellar membrane being released every 6 hrs or less. Brefeldin-A destroyed Chlamydomonas Golgi, inhibited the secretory pathway, inhibited flagellar regeneration, and induced full-length flagella to disassemble within 6 hrs, consistent with flagellar disassembly being induced by a failure to resupply membrane. In contrast to membrane lipids, a pool of biotinylatable membrane proteins was identified that was sufficient to resupply flagella as they released vesicles for 6 hrs in the absence of protein synthesis and to support one and nearly two regenerations of flagella following amputation. These studies reveal the importance of the secretory pathway to assemble and maintain full-length flagella.

Cep164 mediates vesicular docking to the mother centriole during early steps of ciliogenesis

Cep164 provides a molecular link between the mother centriole and the ciliary membrane biogenesis machinery by interacting with the GEF Rabin8 and the GTPase Rab8.

Cilia formation is a multi-step process that starts with the docking of a vesicle at the distal part of the mother centriole. This step marks the conversion of the mother centriole into the basal body, from which axonemal microtubules extend to form the ciliary compartment. How vesicles are stably attached to the mother centriole to initiate ciliary membrane biogenesis is unknown. Here, we investigate the molecular role of the mother centriolar component Cep164 in ciliogenesis. We show that Cep164 was indispensable for the docking of vesicles at the mother centriole. Using biochemical and functional assays, we identified the components of the vesicular transport machinery, the GEF Rabin8 and the GTPase Rab8, as interacting partners of Cep164. We propose that Cep164 is targeted to the apical domain of the mother centriole to provide the molecular link between the mother centriole and the membrane biogenesis machinery that initiates cilia formation.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Targeting proteins to the ciliary membrane

Most vertebrate cell types display solitary nonmotile cilia on their surface that serve as cellular antennae to sense the extracellular environment. These organelles play key roles in the development of mammals by coordinating the actions of a single cell with events occurring around them. Severe defects in cilia lead to midgestational lethality in mice while more subtle defects lead to pathology in most organs of the body. These pathologies range from cystic diseases of the kidney, liver, and pancreas, to retinal degeneration, to bone and skeletal defects, hydrocephaly, and obesity. The sensory functions of cilia rely on proteins localized specifically to the ciliary membrane. Even though the ciliary membrane is a subdomain of the plasma membrane and is continuous with the plasma membrane, cells have the ability to specifically localize proteins to this domain. In this chapter, we will review what is currently known about the structure and function of the ciliary membrane. We will further discuss ongoing work to understand how the ciliary membrane is assembled and maintained, and discuss protein machinery that is thought to play a role in sorting or trafficking proteins to the ciliary membrane.

The intraflagellar transport protein IFT20 is associated with the Golgi complex and is required for cilia assembly

Eukaryotic cilia are assembled via intraflagellar transport (IFT) in which large protein particles are motored along ciliary microtubules. The IFT particles are composed of at least 17 polypeptides that are thought to contain binding sites for various cargos that need to be transported from their site of synthesis in the cell body to the site of assembly in the cilium. We show here that the IFT20 subunit of the particle is localized to the Golgi complex in addition to the basal body and cilia where all previous IFT particle proteins had been found. In living cells, fluorescently tagged IFT20 is highly dynamic and moves between the Golgi complex and the cilium as well as along ciliary microtubules. Strong knock down of IFT20 in mammalian cells blocks ciliary assembly but does not affect Golgi structure. Moderate knockdown does not block cilia assembly but reduces the amount of polycystin-2 that is localized to the cilia. This work suggests that IFT20 functions in the delivery of ciliary membrane proteins from the Golgi complex to the cilium.

Hsp40 is involved in cilia regeneration in sea urchin embryos

In a previous paper we demonstrated that, in Paracentrotus lividus embryos, deciliation represents a specific kind of stress that induces an increase in the levels of an acidic protein of about 40 kD (p40). Here we report that deciliation also induces an increase in Hsp40 chaperone levels and enhancement of its ectodermal localization. We suggest that Hsp40 might play a chaperoning role in cilia regeneration.