(Microbial infection) In case of infection by polyomavirus, involved in the virus endoplasmic reticulum membrane penetration and infection (PubMed:25631089). Regulates the recruitment of DNAJB12:DNAJB14 into SV40-induced foci and all cooperate to guide SV40 across the endoplasmic reticulum membrane. The foci represent the site from which SV40 penetrates into the cytosol (PubMed:25631089). . This is the function of DNAJC18 (DnaJ heat shock protein family (Hsp40) member C18, Ensembl gene identifier ENSG00000170464).