Plakins are large modular proteins that help to maintain tissue integrity by anchoring cytoskeletal filaments to junctional complexes in the membrane, such as desmosomes. Typically, they contain several different domains, which allow them to interact with both the cytoskeleton and membrane-associated proteins. But one member of the family, epiplakin, contains only plakin-repeat domains (PRDs) - motifs that bind to intermediate filaments (IFs). So what is its function? Shyh-Ing Jang and co-workers have used RNAi and a new anti-epiplakin antibody to answer this question (). They show that epiplakin is closely associated with the keratin IF network in cells and preferentially interacts with assembled filaments rather than keratin monomers in vitro. They also demonstrate that knocking down epiplakin by RNAi causes a collapse of the IF network in simple epithelial cells - but, interestingly, not in epidermal cells. The authors propose that this member of the plakin family preserves the integrity of the IF network in these cells by bridging neighbouring IFs. They note that epiplakin levels in keratinocytes increase as the cells differentiate, suggesting this could be because new keratins (K1/K10) are expressed then.
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