Control of the terminal step of intracellular membrane fusion by protein phosphatase 1

Intracellular membrane fusion is crucial for the biogenesis and maintenance of cellular compartments, for vesicular traffic between them, and for exo- and endocytosis. Parts of the molecular machinery underlying this process have been identified, but most of these components operate in mutual recogn ition of the membranes. Here it is shown that protein phosphatase 1 (PP1) i s essential for bilayer mixing, the last step of membrane fusion, PP1 was a lso identified in a complex that contained calmodulin, the second known fac tor implicated in the regulation of bilayer mixing. The PP1-calmodulin comp lex was required at multiple sites of intracellular trafficking; hence, PP1 may be a general factor controlling membrane bilayer mixing.