Proteasome ATPases unravel folded proteins. Introducing a sequence containing only glycine and alanine residues (GAr) into substrates can impair their digestion. We previously proposed that a GAr interferes with the unfolding capacity of the proteasome, leading to partial degradation of products. Here we tested that idea in several ways. Stabilizing or destabilizing a folded domain within substrate proteins changed GAr‐mediated intermediate production in the way predicted by the model. A downstream folded domain determined the sites of terminal proteolysis. The spacing between a GAr and a folded domain was critical for intermediate production. Intermediates containing a GAr did not remain associated with proteasomes, excluding models whereby retained GAr‐containing proteins halt further processing. The following model is supported: a GAr positioned within the ATPase ring reduces the efficiency of coupling between nucleotide hydrolysis and work performed on the substrate. If this impairment takes place when unfolding must be initiated, insertion pauses and proteolysis is limited to the portion of the substrate that has already entered the catalytic chamber of the proteasome.
Kindlin-1 binds to integrins and regulates integrin activation at cell adhesions. Here we report a new function of Kindlin-1 in regulating spindle assembly. We show that Kindlin-1 localizes to centrosomes, its concentration peaking during G2/M, where it associates with various pericentriolar material proteins, including Polo-like kinase 1. Short interfering RNA-mediated depletion of Kindlin-1 increases formation of abnormal mitotic spindles and decreases cellular survival. This effect is dependent not only on the ability of Kindlin-1 to bind integrins but also on Polo-like kinase 1-mediated Kindlin-1 phosphorylation. We demonstrate that a subcellular pool of phosphorylated Kindlin-1 is located exclusively at centrosomes. Our work identifies a novel cellular role for Kindlin-1 in ensuring mitotic spindle assembly and cellular survival that is controlled by phosphorylation via Polo-like kinase 1.
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