CD21 is the cellular receptor for Epstein-Barr virus (EBV). It is expressed by B cells and dendritic cells and has an important role as a co-receptor in B-cell receptor (BCR) signalling. The pathways CD21 activates remain obscure. Joyce Fingeroth and co-workers have therefore searched for proteins that interact with its cytoplasmic region (see ). They find that this region interacts with FHOS – a member of the formin family of actin-nucleating proteins – in two-hybrid studies. They confirm that the interaction occurs in vivo by expressing a GFP-FHOS fusion protein in a B-cell line and showing that the two proteins colocalize and can be coprecipitated. The authors go on to demonstrate that FHOS colocalizes with actin at the cell periphery and is particularly evident in elongating protrusions. Moreover, they show that exposing CD21 to purified EBV or a crosslinking antibody enhances this effect and causes CD21 to redistribute to large membrane patches. Since formins can direct cytoskeletal rearrangements and FHOS is implicated in vesicle trafficking, the interaction could be important both for formation of microdomains in BCR signalling and during endocytosis of EBV.
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