YPXL/I Is a Protein Interaction Motif Recognized by Aspergillus PalA and Its Human Homologue, AIP1/Alix

Abstract

The zinc finger transcription factor PacC undergoes two-step proteolytic activation in response to alkaline ambient pH. PalA is a component of the fungal ambient pH signal transduction pathway. Its mammalian homologue AIP1/Alix interacts with the apoptosis-linked protein ALG-2. We show that both PalA and AIP1/Alix recognize a protein-protein binding motif that we denote YPXL/I, where Tyr, Pro, and Leu/Ile are crucial for its interactive properties. Two such motifs flanking the signaling protease cleavage site mediate direct binding of PalA to PacC, required for the first and only pH-regulated cleavage of this transcription factor. PalA can bind the “closed” (i.e., wild-type full-length) conformer of PacC, suggesting that PalA binding constitutes the first stage in the two-step proteolytic cascade, recruiting or facilitating access of the signaling protease, presumably PalB. In addition to recognizing YPXL/I motifs, both PalA and AIP1/Alix interact with the Aspergillus class E Vps protein Vps32 homologue, a member of a protein complex involved in the early steps of the multivesicular body pathway, suggesting that this interaction is an additional feature of proteins of the PalA/AIP1/Alix family.

ACKNOWLEDGMENTS

We thank T. Suárez for helpful discussions and A. Akintade and E. Reoyo for technical assistance.

We thank the CICYT, the BBSRC, and the EU (through grants BIO2000-920 [to M.A.P.], 60/P11494 [to H.N.A.], and QLK3-CT-1999-00729 [to M.A.P. and H.N.A.]) for support. O.V. held an EMBO long-term fellowship and is currently supported by the Ramón y Cajal Program (MCyT; Spain). L.R. held a BBSRC studentship.