Regulation of Organelle Membrane Fusion by Pkc1p

Abstract

Membrane fusion relies on complex protein machineries, which act in sequence to catalyze the fusion of bilayers. The fusion of endoplasmic reticulum membranes requires the t-SNARE Ufe1p, and the AAA ATPase p97/Cdc48p. While the mechanisms of membrane fusion events have begun to emerge, little is known about how this fusion process is regulated. We provide first evidence that endoplasmic reticulum membrane fusion in yeast is regulated by the action of protein kinase C. Specifically, Pkc1p kinase activity is needed to protect the fusion machinery from ubiquitinmediated degradation.Key words: AAA protein, Cdc48, membrane fusion, phosphorylation, protein kinase C, proteolysis, SNARE, Ubc7, ubiquitination, Ufe1 Received 10 July 2001, revised and accepted for publication 26 July 2001The homotypic fusion of endoplasmic reticulum (ER) membranes is a process essential for normal cell division and maintaining the organelle. In the yeast S. cerevisiae, membrane fusion of the ER and contiguous nuclear envelope must occur during cell division, during nuclear fusion after mating (karyogamy), and for constitutive maintenance of the ER. Although membrane fusion is a process essential for many cellular functions, the detailed series of events by which membranes fuse is still not understood.

Ice2p is important for the distribution and structure of the cortical ER network in Saccharomyces cerevisiae

“…This tubular network is likely to be maintained by ER tubule-tubule fusion (Lin et al, 2001;Prinz et al, 2000). In this study, we have shown that Ice2p is specifically required for the maintenance and distribution of the cortical ER network.…”

Werner syndrome protein directly binds to the AAA ATPase p97/VCP in an ATP-dependent fashion

“…These diverse functions are mediated by different sets of adaptor proteins . Cdc48p and its orthologs form functional complexes with several ER membrane proteins, including the yeast Ufe1p (Latterich et al, 1995;Lin et al, 2001; and Shp1p (Indig and Latterich, unpublished observations) proteins and their vertebrate orthologs, p47 and syntaxin 5 (Acharya et al, 1995;Rabouille et al, 1995). Cdc48p/p97/VCP functions as an adapter system in ERAD and ubiquitin-dependent protein degradation via the cofactor proteins Npl4 and Ufd1 in both yeast and mammals (Braun et al, 2002;Hitchcock et al, 2001;Ye et al, 2001).…”

Distinct Roles for the AAA ATPases NSF and p97 in the Secretory Pathway

“…p97 participates in reassembly of ilimaquinone dispersed Golgi membranes into Golgi stacks (Acharya et al, 1995), reformation of unfenestrated Golgi cisternae from mitotic Golgi fragments (Rabouille et al, 1995(Rabouille et al, , 1998, fusion of yeast ER membranes (Latterich et al, 1995;Lin et al, 2001), formation but not maintenance of tubulated ER from low-density microsomes Roy et al, 2000), and formation and growth of the nuclear envelope and associated ER network (Hetzer et al, 2001). These in vitro assays all reconstitute complex processes related to organelle assembly.…”