U2AF Homology Motifs (UHMs) mediate protein-protein interactions with U2AF Ligand Motifs (ULMs) of pre-mRNA splicing factors. The UHM-containing alternative splicing factor CAPERalpha regulates splicing of tumor-promoting VEGF isoforms, yet the molecular target of the CAPERalpha UHM is unknown. Here, we present structures of the CAPERalpha UHM bound to a representative SF3b155 ULM at 1.7 A resolution, and for comparison, in the absence of ligand at 2.2 A resolution. The prototypical UHM/ULM interactions authenticate CAPERalpha as a bona fide member of the UHM-family of proteins. We identify SF3b155 as the relevant ULM-containing partner of full-length CAPERalpha in human cell extracts. Isothermal titration calorimetry comparisons of the purified CAPERalpha UHM binding known ULM-containing proteins demonstrate that high affinity interactions depend on the presence of an intact, intrinsically-unstructured SF3b155 domain containing seven ULM-like motifs. The interplay among bound CAPERalpha molecules gives rise to the appearance of two high affinity sites in the SF3b155 ULM-containing domain. In conjunction with the previously-identified, UHM/ULM-mediated complexes of U2AF65 and SPF45 with SF3b155, this work demonstrates the capacity of SF3b155 to offer a platform for coordinated recruitment of UHM-containing splicing factors.