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Molecular and Cellular Biology, January 2000, p. 249-260, Vol. 20, No. 1
0270-7306/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

The Rous Sarcoma Virus Env Glycoprotein Contains a Highly Conserved Motif Homologous to Tyrosine-Based Endocytosis Signals and Displays an Unusual Internalization Phenotype

Christina Ochsenbauer, Susan R. Dubay, and Eric Hunter*

Department of Microbiology, University of Alabama at Birmingham, Birmingham, Alabama 35294

Received 2 August 1999/Returned for modification 1 September 1999/Accepted 5 October 1999

The cytoplasmic domains of retroviral transmembrane (TM) glycoproteins contain conserved sequence motifs that resemble tyrosine-based (YXXØ-type) endocytosis signals. We have previously described a mutant Rous sarcoma virus (RSV) Env protein, Env-µ26, with an L165R mutation in the membrane-spanning domain (MSD) of TM, that exhibited dramatically decreased steady-state surface expression (G. L. Davis and E. Hunter, J. Cell Biol. 105:1191-1203, 1987; P. B. Johnston, J. Y. Dong, and E. Hunter, Virology 206:353-361, 1995). We now demonstrate that the tyrosine of the Y190RKM motif in the RSV TM cytoplasmic domain is crucial for the µ26 phenotype and is part of an efficient internalization signal in the context of a mutant MSD. In contrast, despite the presence of the Y190RKM motif, wild-type RSV Env is constitutively internalized at a slow rate (1.1%/min) more characteristic of bulk uptake during membrane turnover than of active clustering into endocytic vesicles. The µ26 mutation and two MSD mutations that abrogate palmitoylation of TM resulted in enhanced Env endocytosis indicative of active concentration into coated pits. Surprisingly, an Env-Y190A mutant was apparently excluded from coated pits since its uptake rate of 0.3%/min was significantly below that expected for the bulk rate. We suggest that in RSV Env an inherently functional endocytosis motif is silenced by a counteracting determinant in the MSD that acts to prevent clustering of Env into endocytic vesicles. Mutations in either the cytoplasmic tail or the MSD that inactivate one of the two counteracting signals would thus render the remaining determinant dominant.

* Corresponding author. Mailing address: Department of Microbiology, University of Alabama at Birmingham, 845 19th St. South, Birmingham, AL 35294. Phone: (205) 934-4321. Fax: (205) 934-1640. E-mail: ehunter{at}uab.edu.

Molecular and Cellular Biology, January 2000, p. 249-260, Vol. 20, No. 1
0270-7306/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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