In eukaryotes, the poly(A)-binding protein (PABP) is one of the important factors for initiation of messenger RNA translation. PABP activity is regulated by the PABP-interacting proteins (Paips), which include Paip1, Paip2A, and Paip2B. Human Paip1 has three different isoforms. Here, we report the crystal structure of the middle domain of Paip1 isoform 2 (Paipl M) as determined by single-wavelength anomalous dispersion phasing. The structure reveals a crescent-shaped domain consisting of 10 alpha-helices and two antiparallel beta-strands forming a beta-hairpin. The 10 alpha-helices are arranged as five HEAT repeats which form a double layer of alpha helices with a convex and a concave surface. Despite low sequence identity, the overall fold of Paipl M is similar to the middle domain of human eIF4GII and yeast eIF4GI. Moreover, the amino-acid sequence motif and the local structure of eIF4G involved in binding of eIF4A, are conserved in Paipl. The structure reported here is the first of a member of the Paip family, thereby filling a gap in our understanding of initiation of eukaryotic mRNA translation in three dimensions. (C) 2011 Elsevier Inc. All rights reserved.