Ceramide Accumulation Uncovers a Cycling Pathway for the cis-Golgi Network Marker, Infectious Bronchitis Virus M Protein

Maceyka, Michael; Machamer, Carolyn E.

The M glycoprotein from the avian coronavirus, infectious bronchitis virus (IBV), 1 is a model protein for studying localization to the early secretory pathway. Immunoelectron microscopy showed that IBV M expressed in the absence of other IBV proteins was found in the tubulovesicular structures at the entry or cis face of the Golgi stack, as well as the first or second cisterna of the Golgi stack (Machamer et al., 1990; Sodeik et al., 1993) . We will refer to this region as the cis -Golgi network (CGN; Mellman and Simons, 1992) . Defined in this way, we would consider the CGN to at least partially overlap with the intermediate compartment (IC), defined by such markers as ERGIC-53 or p58 (Schweizer et al., 1988; Lahtinen et al., 1996) . The first transmembrane domain of IBV M is sufficient to target chimeric proteins to the CGN (Swift and Machamer, 1991; Machamer et al., 1993) . Although the transmembrane domains of other Golgi proteins also contain targeting information, no consensus motif for localization has been identified (for review see Colley, 1997) . We are intrigued by the possibility that the targeting of Golgi membrane proteins may in part depend on interactions between their transmembrane domains and specific membrane lipids.