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Department of Cell Biology and Anatomy, The Johns Hopkins School of Medicine, Baltimore, Maryland 21205
The M glycoprotein from the avian coronavirus, infectious bronchitis virus (IBV), contains information for localization to the cis-Golgi network in its first
transmembrane domain. We hypothesize that localization to the Golgi complex may depend in part on specific interactions between protein transmembrane domains and membrane lipids. Because the site of
sphingolipid synthesis overlaps the localization of IBV
M, we asked whether perturbation of sphingolipids affected localization of IBV M. Short-term treatment
with two inhibitors of sphingolipid synthesis had no effect on localization of IBV M or other Golgi markers.
Thus, ongoing synthesis of these lipids was not required
for proper localization. Surprisingly, a third inhibitor,
d,l-threo-1-phenyl-2-decanoylamino-3-morpholino- 1-propanol (PDMP), shifted the steady-state distribution of IBV M from the Golgi complex to the ER. This
effect was rapid and reversible and was also observed
for ERGIC-53 but not for Golgi stack proteins. At the
concentration of PDMP used, conversion of ceramide
into both glucosylceramide and sphingomyelin was inhibited. Pretreatment with upstream inhibitors partially
reversed the effects of PDMP, suggesting that ceramide
accumulation mediates the PDMP-induced alterations.
Indeed, an increase in cellular ceramide was measured in PDMP-treated cells. We propose that IBV M is at
least in part localized by retrieval mechanisms. Further,
ceramide accumulation reveals this cycle by upsetting
the balance of anterograde and retrograde traffic and/
or disrupting retention by altering bilayer dynamics.
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