Rab27a facilitates human parainfluenza virus type 2 growth by promoting cell surface transport of envelope proteins

Human parainfluenza virus type 2 (hPIV-2) proteins and genomes newly synthesized in the cytoplasm need to be transported to the plasma membrane where budding occurs. This mechanism, where Rab proteins regulate intracellular traffic by switching between GTP-bound active form and GDP-bound inactive form, is not fully understood. mRNA and protein expression levels of Rab8a, Rab11a, and Rab27a are not altered by hPIV-2 infection. hPIV-2 growth is affected by depletion of Rab27a but not Rab8a and Rab11a. Overexpression of a constitutively active mutant of Rab27a Q78L promotes the cell surface levels of fusion (F) and hemagglutinin-neuraminidase (HN) proteins in hPIV-2-infected cells without affecting viral mRNA levels. Increase in the cell surface level of F and HN proteins by Rab27a Q78L is noticeable when these proteins are coexpressed independent of hPIV-2 infection. Our results collectively suggest that the active form of Rab27a enhances hPIV-2 growth by promoting transport of F and HN proteins to the plasma membrane.