CDC42EP1

CDC42EP1 (MSE55/Borg5) is a CRIB-domain effector that links the Rho-family GTPases Cdc42 and TC10 to the actin and septin cytoskeletons, thereby controlling cell shape, contractility, and directed migration (PMID:10430899, PMID:29040749). It binds the GTP-bound forms of Cdc42 and TC10—but not Rac1 or RhoA—through an intact CRIB domain, and a CRIB mutant abolishes both Cdc42 binding and downstream activity (PMID:7493928, PMID:10490598, PMID:10430899). Acting downstream of Cdc42, CDC42EP1 localizes to membrane ruffles and protrusions where it drives actin polymerization and pseudopodia/protrusion formation in a manner blocked by dominant-negative Cdc42 (PMID:10430899, PMID:11035016). CDC42EP1 also partitions to perinuclear and cell-center compartments where it reciprocally interacts with septin filaments: septins recruit CDC42EP1 to the cell center while CDC42EP1 supports septin filament assembly, and this interplay governs actin stress fiber stability, orientation, and contractility (PMID:29040749, PMID:36923257). Through a physical interaction with the rod domain of myosin IIA (MYH9) that is antagonized by septins, CDC42EP1 restrains actomyosin contractility, cell-cell adhesion tension, and leading-edge dynamics in epithelial cells (PMID:39503295). These activities place CDC42EP1 in developmental and morphogenetic programs including neural crest cell migration and trophectoderm differentiation, the latter via interaction with atypical PKC (PMID:20506138, PMID:29040749).

1. 1995 Medium

   Established CDC42EP1 as a GTPase effector by showing it carries a CRIB motif and binds GTP-loaded Cdc42 selectively over RhoA, defining its specificity from the outset.

   Evidence Filter binding assay with GST-fused GTPases

   PMID:7493928

   Open questions at the time

   • Weaker Rac binding left selectivity incompletely resolved
   • No functional consequence of binding tested
   • Single in vitro binding format
2. 1998 Medium

   Broadened the GTPase specificity by placing CDC42EP1 in the shared effector set of TC10 in addition to Cdc42.

   Evidence Yeast two-hybrid and/or pulldown interaction assay

   PMID:9799731

   Open questions at the time

   • GTP-dependence of TC10 binding not yet established here
   • No cellular function attributed to TC10 binding
3. 1999 High

   Defined the molecular requirements and consequences of effector binding: GTP- and CRIB-dependent engagement of Cdc42/TC10 drives membrane ruffle localization, actin polymerization, and Cdc42-dependent protrusions, formally establishing CDC42EP1 as a Cdc42 effector for actin remodeling.

   Evidence Yeast two-hybrid, GST pulldown, CRIB mutagenesis, dominant-negative epistasis, immunofluorescence and live imaging in Cos-7/NIH 3T3 cells

   PMID:10430899 PMID:10490598

   Open questions at the time

   • Direct biochemical link between binding and actin nucleation machinery unresolved
   • No endogenous loss-of-function data
4. 2001 Medium

   Generalized the actin-remodeling activity across the CEP/Borg family, confirming CRIB- and Cdc42-dependent pseudopodia induction.

   Evidence GST pulldown, CRIB mutagenesis, dominant-negative co-expression, actin immunofluorescence in NIH 3T3

   PMID:11035016

   Open questions at the time

   • CDC42EP1-specific CRIB requirement inferred from paralog CEP2 mutagenesis
   • Effector output mechanism still phenomenological
5. 2010 Medium

   Connected CDC42EP1 to a developmental program by showing it acts downstream of Cdc42 with aPKC to drive trophectoderm motility and sorting during blastocyst formation.

   Evidence Co-immunoprecipitation, morpholino knockdown, live-cell imaging and aPKC immunofluorescence in differentiating ESCs

   PMID:20506138

   Open questions at the time

   • Mechanism linking Borg5 to aPKC localization unresolved
   • Single loss-of-function approach
6. 2018 High

   Resolved how CDC42EP1 controls migration directionality by linking its dual localization—Cdc42-positive protrusions versus Cdc42-absent perinuclear patches—to traction force generation and membrane protrusion in neural crest cells.

   Evidence Morpholino knockdown, Co-IP, live imaging, focal adhesion immunofluorescence, traction force microscopy in frog embryos

   PMID:29040749

   Open questions at the time

   • Identity of factors retaining CDC42EP1 at perinuclear patches unknown
   • Cross-species generality to mammalian migration not tested
7. 2023 Medium

   Established a reciprocal CDC42EP1–septin module that controls actin stress fiber stability, orientation, and contractility, and showed CDC42EP1 also scaffolds septins around invading Salmonella in a Cdc42-dependent manner relevant to intracellular bacterial growth.

   Evidence Morpholino knockdown and rescue with live imaging in neural crest cells; CDC42 interactome proteomics, siRNA, and infection assays in enterocytes

   PMID:36923257 PMID:37877586

   Open questions at the time

   • Molecular basis of reciprocal septin recruitment not defined
   • Whether SEPTIN-7 binding is direct unresolved
   • Role at Salmonella-containing vacuoles mechanistically uncharacterized
8. 2024 Medium

   Identified a contractility-limiting mechanism: CDC42EP1 binds the myosin IIA rod domain, an interaction antagonized by septins, allowing it to restrain actomyosin contractility, adhesion tension, and leading-edge dynamics in epithelia.

   Evidence siRNA depletion, Co-IP, immunofluorescence, live imaging, traction/adhesion tension measurements in MDCK cells

   PMID:39503295

   Open questions at the time

   • Direct vs indirect MYH9 rod-domain binding not separated from septin competition
   • Structural basis of the septin-myosin switch unknown

• How CDC42EP1 mechanistically integrates Cdc42 GTPase input, septin assembly, and myosin IIA regulation into a single switch governing protrusion versus contractility remains unresolved.
• No structural model of the CRIB–GTPase or rod-domain interfaces
• Direct vs scaffolded nature of septin and myosin interactions undefined
• Endogenous mammalian loss-of-function phenotypes limited