Correction of cell–cell communication defect by introduction of a protein kinase into mutant cells

Abstract

The cell-to-cell permeability of the junctions of various cultured mammalian cell types depends on the concentration of intracellular cyclic AMP ([cAMP]_i). The permeability rises when the cells are supplied with exogenous cyclic AMP or when their cyclic AMP synthesis is stimulated with choleragen or hormones; it falls when [cAMP]_i is lowered by application of serum or due to increase in cell density^1–5. The rise and fall in permeability take several hours to develop (the rise is protein synthesis-dependent) and they occur concurrently with the rise and fall in the number of intramembrane particles of the gap junctions^2–4, which probably embody the cell-to-cell channels. Is this permeability regulation mediated by phosphorylating protein kinase? In many eukaryotes, the cyclic AMP receptor is a protein kinase^6–8 consisting of a pair of regulatory subunits and a pair of catalytic subunits. The latter dissociate from the holoenzyme as the cyclic AMP binds to the regulatory subunits and, in this dissociated form, catalyse the phosphorylation of the target⁹. The regulatory subunit occurs in two isoenzyme forms, I and II. The catalytic subunit seems invariant⁹; subunits from different isoenzymes can substitute for each other^10,11. We show here that a mutant cell lacking the isoenzyme I is deficient in permeable junctions, and that this junctional defect is corrected when the mutant is supplied with exogenous catalytic subunit.