Protein phosphatase 1 delta is associated with focal adhesions

In all mammalian cells protein phosphatase-1 (PP1) exists in three isoforms, defined as alpha, gamma 1 and delta. Immunofluorescence studies with isoform-specific antibodies indicated that delta, but not alpha or gamma 1, is enriched at focal adhesions in HeLa cells, fibroblasts, endothelial cells and keratinocytes. This was confirmed also by interference reflection microscopy, which indicated that PP1 delta 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 PP1 delta in focal adhesions represented only a small aliquot of the total PP1 delta, which was predominantly localized to the nucleus. The association of PP1 delta to focal adhesions was confirmed by the co-immunoprecipitation of PP1 delta with the focal adhesion kinase pp125FAK and with the alpha v integrin. Comparison between the amount of PP1 delta associated with focal adhesion proteins and that of PP1 delta recovered in an anti-PP1 delta 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 PP1 delta may be involved in the regulation of focal adhesion functions and particularly in the signaling pathway generated by cell-substratum adhesion.