Association of ribosomal subunits

A yeast ribosomal subunit association factor (AF) has been purified from a high‐salt ribosomal wash. The purified enzyme is a thermostable protein that associates ribosomal subunits at low Mg2+ concentration without requiring energy. It appears to be an aggregate of trimers or dimers (molecular mass 125 or 79 kDa) which on sodium dodecyl sulfate gels shows the presence of a major protein band whose estimated molecular mass is 43 kDa. Evidence also indicates the existence of a 50‐kDa polypeptide which seems to be unstable since with freezing and thawing it gives rise to the 43‐kDa polypeptide. It was shown that the labelled factor interacts with 80S ribosomes and with 40S ribosomal subunits. The purified polypeptide reacts with antibodies directed against EF‐1α, this last protein recognizing the antibodies raised against AF. Likewise, both EF‐1α and AF associate ribosomal subunits in the same way. When EF‐1 is heated, it not only maintains its association activity, but also behaves like a 43‐kDa polypeptide in an SDS electrophoresis run. These observations strongly suggest that AF originates from EF‐1α, which implies that the well‐known elongation factor may also play a role in the initiation step of protein synthesis.