Branched actin as a cell cycle checkpoint?
Irene Dang and Alexis Gautreau hypothesize that the branched actin network might act as a signal for cell cycle progression through G1. Interestingly, only branched actin from lamellipodia, but not from endocytic pits or endosomes, seems to be sensed. This might be achieved via activation of the WAVE pathway, which is already well known to be required for the formation of membrane protrusions. Furthermore, the authors discuss the implications of this branched actin checkpoint for cancer cells.
Dang and Gautreau, Bioessays 2012;34: 1021–1024.
Synergy of mutations
Sortase A (SrtA) from Staphylococcus aureus has an important role: to display virulence factors on the cell wall envelope. The enzyme cleaves the peptide bond between Thr and Gly in the LPXTG motif and can re-form the peptide bond with an exogenous oligoglycine. S. sureus SrtA is now a powerful tool for post-translational protein modification such as selective labeling of cell-surface proteins on living cells. However, the applications are limited to Ca2+-containing conditions because Ca2+ stabilizes the substrate binding site. Hirakawa, Ishikawa, and Nagamune demonstrate that mutations of the Ca2+-binding site conferred Ca2+-independent activity to S. sureus SrtA. Interestingly, either substitution of two Glu residues in the Ca2+-binding site did not improve the activity, while combinations of the single mutations significantly improved it in the absence of Ca2+. Further work by the authors is underway to selectively label cytosolic proteins in living cells by the Ca2+-independently active mutants.
Hirakawa, Ishikawa, and Nagamune, Biotechnol. Bioeng. 2012;109:2955–2961.