Proteins associated with poly(A)+RNA of cockerel liver: effects of estradiol stimulation.

The protein population of poly(A)-containing messenger ribonucleoprotein (mRNP) from cockerel liver was analyzed before and after estradiol induction. Polysomal and free mRNP were isolated by thermal elution from oligo(dT)-cellulose. The proteins were separated by two-dimensional electrophoresis (of which the first dimension was a nonequilibrium pH gradient) and were visualized by silver staining. In order to determine similarities and differences between proteins of various mRNP fractions, trace amounts of 125I-labeled protein from one fraction were coelectrophoresed with stainable amounts of protein from another fraction on the same gel. Of 27 proteins analyzed, 15 were common to polysomal and free mRNP, 7 were specific to polysomal mRNP, and 5 were specific to free mRNP. Moreover, estradiol strongly influenced 11 proteins. Six proteins changed either in relative intensity or in distribution between polysomal and free mRNP fractions. Three major proteins appeared in both fractions, and two additional proteins disappeared from the free mRNP fraction after estradiol treatment. The results suggest that the protein population in polysomal mRNP is quite complex and that the profound influence of estradiol on protein synthesis in cockerel liver may be connected to changes in the protein composition of mRNP.