The spike protein of Infectious bronchitis virus is intracellularly retained by a tyrosine motif

Christine Winter^*, Christel Schwegmann-Weels, Ulrich Neumann, and Georg Herrler

Institute of Virology, and Clinic for Poultry, University of Veterinary Medicine, Hannover, Bünteweg 17, 30559 Hannover, Germany

^* To whom correspondence should be addressed. Email: Christine.Winter{at}tiho-hannover.de.

	Abstract

We have analyzed the intracellular transport of the S protein of Infectious bronchitis virus, an avian coronavirus. Surface expression was analyzed by immunofluorescence microscopy, by surface biotinylation, and by syncytia formation of S-expressing cells. Applying these methods, the S protein was found to be retained intracellularly. Tyr1143 in the cytoplasmic tail was shown to be a crucial component of the retention signal. Deletion of a di-lysine motif that has previously been suggested to function as a retrieval signal did not abolish intracellular retention. Treatment of the S proteins with endoglycosidases did not reveal any differences between the parental and the mutant proteins. Furthermore, all S proteins analyzed were posttranslationally cleaved into the subunits S1 and S2. In co-expression experiments, the S protein was found to colocalize with a Golgi marker. Taken together, these results indicate that the S protein of IBV is retained at a late Golgi compartment. Therefore, this viral surface protein differs from the S proteins of Transmissible gastroenteritis virus and SARS-CoV that are retained at a pre-Golgi compartment or transported to the cell surface, respectively. The implications of these differences are discussed.