Cap binding protein (CBP)-related polypeptides were identified in different cytoplasmic RNP particles of embryonic chick muscles using monoclonal antibody to purified CBP. A single immunoreactive peptide (M r 78000) was present in preparations of both free mRNP particles and a novel 10 S translation inhibitory RNP particle. In contrast, proteins isolated from these particles showed two new low-M r immunoreactive peptides (M r 43000 and M r 29000). No CBP related protein could be detected in polysomal mRNP, although an immunoreactive M r 43000 CBP-related protein was present in polysomes. The relevance of the association of different CBP-related polypeptides with cytoplasmic RNP particles and polysomes are discussed.
