Half-life of the Rous sarcoma virus transforming protein pp60src and its associated kinase activity

Regulation of connexin degradation as a mechanism to increase gap junction assembly and function

Connexins, the integral membrane protein constituents of gap junctions, are degraded at a rate (t(12) = 1.5-5 h) much faster than most other cell surface proteins. Although the turnover of connexins has been shown to be sensitive to inhibitors of either the lysosome or of the proteasome, how connexins are targeted for degradation and whether this process can be regulated to affect intercellular communication is unknown. We show here that reducing connexin degradation with inhibitors of the proteasome (but not with lysosomal blockers) is associated with a striking increase in gap junction assembly and intercellular dye transfer in cells inefficient in both processes under basal conditions. The effect of proteasome inhibitors on wild-type connexin stability, assembly, and function was mimicked by treatment of assembly-inefficient cells with inhibitors of protein synthesis such as cycloheximide. Sensitivity of connexin degradation to cycloheximide, but not to proteasome inhibitors, was abolished when connexins were rendered structurally abnormal by perturbation of essential disulfide bonds or by mutation. Our findings provide the first evidence that intercellular communication can be up-regulated at the level of connexin turnover and that a short-lived protein may be required for conformationally mature connexins to become substrates of proteasomal degradation.

Evidence for regulation of actin synthesis in cytochalasin D-treated HEp-2 cells

In HEp-2 cells treated with 0.2 or 2.0 microM cytochalasin D (CD), the relative rate of actin synthesis increased for about 12 h and then reached a plateau; this increase was suppressed by actinomycin D (AD). When CD was washed from cells which had been treated for 20 h, the elevated rate of actin synthesis declined to the control value within ca 4 h, as the actin-containing cytoskeletal components rearranged by CD recovered their normal morphology. Subsequently, actin synthesis was depressed below control values for a prolonged period; during recovery from 2 h treatment with CD, this depression was of much shorter duration. Re-addition of CD to cells after a 3 h recovery period again induced the cytoskeletal alterations characteristic of CD treatment but did not reverse the prior decline in the rate of actin synthesis. In HEp-2 cells treated with cycloheximide during exposure to CD for 20 h, the relative rate of actin synthesis measured after removal of cycloheximide was twofold higher than with CD alone and such cells exhibited a twofold slower decline in the rate of actin synthesis during recovery from CD in the continued presence of cycloheximide. These effects of cycloheximide, which resemble observations on "super-induction", suggest that actin synthesis in CD-treated and recovering HEp-2 cells may be regulated by a repressor protein. The possibility that the proposed repressor protein is actin and that actin may thus be a feedback inhibitor of its own synthesis is discussed.

Phosphotyrosine-containing proteins and expression of transformation parameters in cells infected with partial transformation mutants of Rous sarcoma virus

We have examined the phosphorylation state of five proteins known to become phosphorylated on tyrosine during transformation by Rous sarcoma virus by using cells infected with a panel of partially transforming mutant viruses. Situations of viral mutant and growth temperature were found in which phosphorylation of some proteins occurred more extensively than that of others, indicating that mutations in the src gene had affected the specificity of pp60src for some of its substrates as well as affecting the activity of the enzyme. To obtain insight into the biological functions of these phosphorylations, comparisons were made between the degree of phosphorylation of these proteins and the expression of various indicators of the transformed phenotype. The data suggest that phosphorylation of proteins l, p, and q (Mr of 46,000, 39,000 and 28,000, respectively) is not sufficient to induce changes in adhesiveness, hexose transport or morphology. The phosphorylation of protein p or l or total phosphotyrosine content correlated well with the production of plasminogen activator, and the phosphorylation of proteins l and q correlated well with increased hexose transport. However, even when good correlations were observed, significant exceptions were sometimes noted. It thus remains possible that some phosphorylations on tyrosine observed in Rous sarcoma virus-transformed cells are not causally related to the expression of the measured parameters of transformation.

Phosphorylation of tyrosine-416 is not required for the transforming properties and kinase activity of pp60v-src

A mutant in src, the oncogene of Rous sarcoma virus, has been constructed in which the major phosphorylated tyrosine (Tyr-416, located in the carboxy-terminal half of the protein) has been replaced by phenylalanine. Mouse cells transformed with this mutant src form foci and grow in soft agar, indicative of a transformed state. Also, the mutant protein retains the wild-type ability to phosphorylate proteins on tyrosine. Partial proteolysis revealed that the carboxy-terminal half of the mutant protein was still phosphorylated, although apparently to a lesser extent. Analysis indicated that this residual phosphorylation was on tyrosine. We conclude that the major tyrosine phosphorylation in pp60v-src is not required for two of the protein's notable properties--protein kinase activity and transformation of cultured cells.