The Interaction of CCDC104/BARTL1 with Arl3 and Implications for Ciliary Function

Abstract

SummaryCilia are small antenna-like cellular protrusions critical for many developmental signaling pathways. The ciliary protein Arl3 has been shown to act as a specific release factor for myristoylated and farnesylated ciliary cargo molecules by binding to the effectors Unc119 and PDE6δ. Here we describe a newly identified Arl3 binding partner, CCDC104/CFAP36. Biochemical and structural analyses reveal that the protein contains a BART-like domain and is called BARTL1. It recognizes an LLxILxxL motif at the N-terminal amphipathic helix of Arl3, which is crucial for the interaction with the BART-like domain but also for the ciliary localization of Arl3 itself. These results seem to suggest a ciliary role of BARTL1, and possibly link it to the Arl3 transport network. We thus speculate on a regulatory mechanism whereby BARTL1 aids the presentation of active Arl3 to its GTPase-activating protein RP2 or hinders Arl3 membrane binding in the area of the transition zone.

Allosteric regulation of Arf GTPases and their GEFs at the membrane interface

“…As predicted from sequence signatures, 22 structural studies of Arf-like and Arf-related GTPases, including Golgi Arl1, 74,75 cilium Arl3 76,77,78 and Arl6 79 and endoplasmic reticulum Sar1 80,81 have now shown that autoinhibition by the N-terminal extension and the interswitch takes place in these GTPases and is released by the displacement of the N-terminus and the toggle of the interswitch. Thus, the allosteric mechanism that uses the displacement of the N-terminal extension as a priming event to autoinhibition release and the remodeling of the interswitch as the means whereby information is propagated to the nucleotide-binding site is probably general to the entire Arf-like family.…”

“…The N-termini of Arfrelated GTPases vary in length, sequence and post-translational modifications, and there is therefore ample reasons to believe that they are displaced in different ways to release autoinhibition. For instance, the N-terminus of Arl3-GTP interacts with another protein, 77,78,82 and it is conceivable that there exists Arf-like GTPases in which autoinhibition release involves interactions of the Nterminus with both a protein and a membrane. Understanding this process will be a major theme for future investigations of the molecular mechanisms that regulate the activity of Arf-like GTPases.…”

Sorting of lipidated cargo by the Arl2/Arl3 system

“…6,7 A homolog of BART, originally identified as CCDC104 (coiled-coiled domain containing 104), now called BARTL1, is much longer than BART but has a domain very similar to the latter. 8 Both BART and BARTL1 bind to the switch regions and also to the amphiphatic helix of the proteins in a similar way. 8,9 The second type of effectors (type2) which bind to both Arl2 and Arl3 are the carrier proteins PDE6d 10 and HRG4/Unc119a 11 and Unc119b.…”

“…8 Both BART and BARTL1 bind to the switch regions and also to the amphiphatic helix of the proteins in a similar way. 8,9 The second type of effectors (type2) which bind to both Arl2 and Arl3 are the carrier proteins PDE6d 10 and HRG4/Unc119a 11 and Unc119b. 12 These proteins have a b-sandwich fold and bind only to the switch regions of Arl2 and Arl3.…”

“…The binding of type 1 and type 2 effectors is nevertheless mutually exclusive. 8,9,[13][14][15] Functional differences between Arl2 and Arl3…”