Krit 1 Interactions With Microtubules and Membranes Are Regulated by Rap1 and Integrin Cytoplasmic Domain Associated protein-1

FEBS J. 2007 Nov;274(21):5518-32. doi: 10.1111/j.1742-4658.2007.06068.x. Epub 2007 Oct 3.

Abstract

The small G protein Rap1 regulates diverse cellular processes such as integrin activation, cell adhesion, cell-cell junction formation and cell polarity. It is crucial to identify Rap1 effectors to better understand the signalling pathways controlling these processes. Krev interaction trapped 1 (Krit1), a protein with FERM (band four-point-one/ezrin/radixin/moesin) domain, was identified as a Rap1 partner in a yeast two-hybrid screen, but this interaction was not confirmed in subsequent studies. As the evidence suggests a role for Krit1 in Rap1-dependent pathways, we readdressed this question. In the present study, we demonstrate by biochemical assays that Krit1 interacts with Rap1A, preferentially its GTP-bound form. We show that, like other FERM proteins, Krit1 adopts two conformations: a closed conformation in which its N-terminal NPAY motif interacts with its C-terminus and an opened conformation bound to integrin cytoplasmic domain associated protein (ICAP)-1, a negative regulator of focal adhesion assembly. We show that a ternary complex can form in vitro between Krit1, Rap1 and ICAP-1 and that Rap1 binds the Krit1 FERM domain in both closed and opened conformations. Unlike ICAP-1, Rap1 does not open Krit1. Using sedimentation assays, we show that Krit1 binds in vitro to microtubules through its N- and C-termini and that Rap1 and ICAP-1 inhibit Krit1 binding to microtubules. Consistently, YFP-Krit1 localizes on cyan fluorescent protein-labelled microtubules in baby hamster kidney cells and is delocalized from microtubules upon coexpression with activated Rap1V12. Finally, we show that Krit1 binds to phosphatidylinositol 4,5-P(2)-containing liposomes and that Rap1 enhances this binding. Based on these results, we propose a model in which Krit1 would be delivered by microtubules to the plasma membrane where it would be captured by Rap1 and ICAP-1.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Motifs
  • Animals
  • Binding Sites
  • Cricetinae
  • Intracellular Signaling Peptides and Proteins / metabolism*
  • Membrane Proteins / metabolism*
  • Membranes / metabolism
  • Microtubule-Associated Proteins / chemistry
  • Microtubule-Associated Proteins / metabolism*
  • Microtubules / metabolism*
  • Models, Biological
  • Models, Molecular
  • Protein Conformation
  • Transfection
  • Two-Hybrid System Techniques
  • rap1 GTP-Binding Proteins / metabolism*

Substances

  • ITGB1BP1 protein, human
  • Intracellular Signaling Peptides and Proteins
  • Membrane Proteins
  • Microtubule-Associated Proteins
  • rap1 GTP-Binding Proteins