SIGNAL-TRANSDUCING AND LOW-MOLECULAR-WEIGHT GTP-BINDING PROTEINS OF LUNG AND ENDOTHELIUM - LOCALIZATION IN MEMBRANES AND CYTOSOL, AND INTERACTION WITH F-ACTIN

The following proteins have been identified in mammalian lung and vasc ular endothelium using [P-32]ADP-ribosylation by bacterial ADP ribosyl transferases and immuno- and[alpha-P-32]GTP-blotting. 41 kD G(i)1alph a, 40 kD G(i)2alpha, 41 kD G(i)3alpha, 40 and 45 kD subunits of G(s)al pha 36 kD beta1- and 35 kD beta2-subunits of signal-transducing GTP-bi nding proteins (G proteins), and also the low molecular weight (19-26 kD) GTP-binding proteins (SMG proteins) ras, rho, rac, G25K (G(p)) and the ARF and SMG proteins that display high binding affinity for [alph a-P-32]GTP. These G and SMG proteins were unevenly distributed between the membrane and cytosol fractions. G(i)2alpha- and G(s)alpha-subunit s, but not beta1-subunit or SMG proteins, could be partially (approxim ately 1%) released from membranes into solution by GTP analogs GTPgamm aS or Gpp(NH)p in the presence of Mg2+. Extraction by low ionic streng th buffers in the presence of EDTA liberates the pertussis toxin-sensi tive G(i)2alpha- and beta1-subunits. Functionally coupled alphabetagam ma heterotrimeric cytosolic subunits of G(i) proteins (predominantly t he G(i)2alpha and beta1) and SMG proteins detected by [alpha-P-32]GTP blotting but not those sensitive to botulinic C3 exoenzyme (rho/rac) o r ARF interact with F-actin. Approximately 20% of these proteins were found in Triton X-100-insoluble (cytoskeletal) endothelial fraction. T herefore, it is conceivable that formation of ''polydisperse'' structu res by cellular G and SMG proteins is directed by their interactions w ith actin filaments.