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Identification of two KH domain proteins in the alpha‐globin mRNP stability complex.

Accumulation of globin mRNAs during erythroid differentiation is dependent on their extraordinary stability. The longevity of human alpha‐globin mRNA is associated with a ribonucleoprotein complex (alpha‐complex) formed on the 3′ untranslated region (3′UTR). One or more of the proteins within this alpha‐complex contain strong polycytosine [poly(C)] binding (alpha PCB) activity. In the present report we purify alpha PCB activity from human erythroid K562 cells. Although not able to bind the alpha‐globin 3′UTR directly, alpha PCB activity is sufficient to complement alpha‐complex formation in a cytosolic extract depleted of poly(C) binding activity. Peptide microsequencing demonstrates that alpha PCB activity contains two structurally related poly(C) binding proteins. These two proteins, alpha‐complex protein (alpha CP)‐1 and −2, have an overall structural identity of 80% and contain three repeats of the K homology (KH) domain which is found in a subset of RNA binding proteins. Epitope‐tagged recombinant alpha CP‐1 and alpha CP‐2 expressed in cells are each incorporated into the alpha‐complex. We conclude that alpha CP‐1 and alpha CP‐2, members of the KH domain RNA binding protein family, are involved in formation of a sequence‐specific alpha‐globin mRNP complex associated with alpha‐globin mRNA stability. As such this represents the first example of a specific function for this class of proteins and suggests potential roles for other members of this protein family.