Spread the word

Using a mass-spectrometry-based technique to uncover proteins that are present in focal adhesions, Matthias Mann and colleagues have identified an early cell-spreading structure — the spreading initiation centre (SIC) — that contains RNA and RNA-binding proteins. Their findings are reported in Cell.

The technique — stable isotope labelling by amino acids in culture (SILAC) — 'labels' proteins by culturing them in medium that contains deuterium-substituted leucine or ¹³C-substituted arginine. These proteins can then be distinguished by mass spectrometry from non-labelled proteins that have been isolated from cells grown in normal medium. The authors applied the technique to immunoprecipitates from lysates of adherent and non-adherent fibroblasts using antibodies against the important focal-adhesion proteins talin, paxillin or vinculin. Many other known focal-adhesion proteins were identified, but some new candidates emerged.

During their characterization of one differentially expressed protein, RACK1 (receptor for activated C kinase-1), Mann and colleagues found that cells underwent at least three different stages of spreading after initial attachment. Circular patches — which the authors called 'spreading initiation centres' — were present in the early stages but became less abundant as cells spread and focal adhesions formed, and disappeared when the focal adhesions were established. Surrounding these SICs was a sheath of actin, which disappeared as the mature focal adhesion became an attachment point for actin stress fibres. Interestingly, SICs were only observed in fibroblasts and non-transformed cells, and on physiological substrates such as fibronectin.

Surprisingly, the authors discovered that several RNA-binding proteins, such as hnRNP K, hnRNP E1, FUS/TLS, Sm B and Sm D, preferentially bound to either talin, paxillin or vinculin in adherent cells. Not only did these proteins localize to SICs but so, too, did RNA. Neither, though, were present in mature focal adhesions. Finally — and perhaps most surprisingly — disrupting the function of hnRNP K, hnRNP E1 and FUS/TLS using antibodies increased the rate of fibroblast spreading. It's certainly something to talk about.