A 52 kDa protein (p52) was identified, using differential extraction and electrophoretic criteria, as a major extracellular and substrate-associated component of normal rat kidney (NRK) fibroblasts. Cells transformed with Kirsten murine sarcoma virus (KNRK cells) did not express p52 constitutively, but were inducible for both p52 production and its substrate association during culture in sodium butyrate (NaB)-supplemented growth medium. Comparative analysis of the relative molecular mass, subcellular distribution, and isoelectric complexity (five variants ranging in pI from 5.4 to 6.2) of the 52 kDa species constitutively and inducibly expressed by NRK and KNRK/NaB cells respectively, indicated that they were, indeed, the same protein. p52 selectively localized to cellular fractions enriched in substrate focal contact sites and associated ventral undersurface components. NaB induction of p52 in KNRK cells occurred before cell spreading; other polar compounds, such as dimethyl sulphoxide, which did not induce KNRK cell spreading, similarly failed to elicit p52 production. p52 accumulated more rapidly in (and was quickly released from) the focal-contact-enriched protein fraction of NRK cells compared with its time course of appearance in the medium. These data collectively suggest that p52 is one of a relatively small number of proteins the synthesis of which is either involved in determination of cell shape or regulated as a consequence of cell-shape changes.

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