Protein Phosphatase 1 δ is Associated with Focal Adhesions

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Abstract
In all mammalian cells protein phosphatase-1 (PP1) exists in three isoforms, defined as α, γ1 and δ. Immunofluorescence studies with isoform-specific antibodies indicated that δ, but not α or γ1, is enriched at focal adhesions in HeLa cells, fibroblasts, endothelial cells and keratinocytes. This was confirmed also by interference reflection microscopy, which indicated that PP1δ was in areas of tight adhesion of the membrane to the extracellular matrix at sites where the microfilament cytoskeleton is organized. In all the cell types so far considered the PP1δ in focal adhesions represented only a small aliquot of the total PP1δ, which was predominantly localized to the nucleus. The association of PP1δ to focal adhesions was confirmed by the co-immunoprecipitation of PP1δ with the focal adhesion kinase pp125FAK and with the αv integrin. Comparison between the amount of PP1δ associated with focal adhesion proteins and that of PP1δ recovered in an anti-PP1δ immunoprecipitate confirmed that only a minor amount of the enzyme was associated with the focal adhesions. Since some focal adhesion proteins are phosphorylated on Ser/Thr, it is likely that PP1δ may be involved in the regulation of focal adhesion functions and particularly in the signaling pathway generated by cell-substratum adhesion.