Abstract
Actin and tropomyosin, purified from both muscle and brain, and α‐actinin, purified from muscle, have been labeled in vitro by reductive methylation to specific activities of greater than 10^5^ dpm/μg protein. Actin so modified bound DNase I and polymerized identically to unmodified actin. Furthermore, the spectral properties of actin did not change after labeling. The interactions of labeled tropomyosin and α‐actinin with F‐actin were nearly identical to those of the unmodified proteins. These modified proteins comigrated with their unmodified counterparts in both SDS‐containing polyacrylamide gels and isoelectric focusing gels. The labeled actin was quantitatively extracted from SDS‐containing polyacrylamide gels (yield > 98% of radioactivity applied demonstrating that all of the radioactivity was protein bound. The reductive methylation procedure worked well at pH 8.0–8.5 in either pyrophosphate buffer or Bicine buffer using formaldehyde with [^3^H]‐sodium borohydride as the reducing agent. The procedure could also be performed at pH 7.0 in phosphate buffer using [^14^C]‐formaldehyde with sodium cyanoborohydride as the reducing agent. Proteins so labeled are ideal for use in quantitative experiments involving protein‐protein interactions.