Internalization of UT-A1 urea transporter is dynamin dependent and mediated by both caveolae- and clathrin-coated pit pathways

Huang H, Feng X, Zhuang J, Frohlich O, Klein JD, Cai H, Sands JM, Chen G. Internalization of UT-A1 urea transporter is dynamin dependent and mediated by both caveolae- and clathrin-coated pit pathways. Am J Physiol Renal Physiol 299: F1389-F1395, 2010. First published September 22, 2010; doi:10.1152/ajprenal.00718.2009.-Dynamin is a large GTPase involved in several distinct modes of cell endocytosis. In this study, we examined the possible role of dynamin in UT-A1 internalization. The direct relationship of UT-A1 and dynamin was identified by coimmunoprecipitation. UT-A1 has cytosolic NH2 and COOH termini and a large intracellular loop. Dynamin specifically binds to the intracellular loop of UT-A1, but not the NH2 and COOH termini. In cell surface biotinylation experiments, coexpression of dynamin and UT-A1 in HEK293 cells resulted in a decrease of UT-A1 cell surface expression. Conversely, cells expressing dynamin mutant K44A, which is deficient in GTP binding, showed an increased accumulation of UT-A1 protein on the cell surface. Cell plasma membrane lipid raft fractionation experiments revealed that blocking endocytosis with dynamin K44A causes UT-A1 protein accumulation in both the lipid raft and nonlipid raft pools, suggesting that both caveolae- and clathrin-mediated mechanisms may be involved in the internalization of UT-A1. This was further supported by 1) small interfering RNA to knock down either caveolin-1 or mu 2 reduced UT-A1 internalization in HEK293 cells and 2) inhibition of either the caveolae pathway by methyl-beta-cyclodextrin or the clathrin pathway by concanavalin A caused UT-A1 cell membrane accumulation. Functionally, overexpression of dynamin, caveolin, or mu 2 decreased UT-A1 urea transport activity and decreased UT-A1 cell surface expression. We conclude that UT-A1 endocytosis is dynamin-dependent and mediated by both caveolae- and clathrin-coated pit pathways.