Arfs on the Golgi: four conductors, one orchestra

Abstract

Arfs are small Ras-superfamily proteins important for regulating membrane trafficking including the recruitment of vesicular coats as well as a diverse range of other functions. There are five Arfs in humans: two Class I Arfs (Arf1 and Arf3), two Class II Arfs (Arf4 and Arf5) and one Class III Arf (Arf6), with Class I and Class II Arfs present on the Golgi apparatus among other locations. These Golgi Arfs (Arf1, Arf3, Arf4 and Arf5) are highly similar in sequence, and knockout studies have established a complex pattern of redundancy, with Arf4 alone able to support cell survival in tissue culture. Moreover, adding to the complexity, functions of Arfs on distinct membranes can involve non-overlapping sets of effectors (e.g., COPI on cis-Golgi membranes and clathrin adaptors on trans-Golgi network). The three classes of Arfs are found in most metazoans, suggesting biologically important specialization the details of which are beginning to emerge. This review examines recent studies using siRNA and CRISPR/Cas9 knockouts of mammalian Arfs combined with functional assays of the secretory pathway in the context of detailed localization of fluorescently-tagged Arfs by fluorescent and super-resolution microscopy and the existing literature using more conventional techniques. We suggest that specificity of effector recruitment involves additional membrane determinants which need to be considered in future studies.

Keywords: ARF; ARF1; ARF3; ARF4; COPI; golgi; redundancy; secretory pathway.

FIGURE 1

Intracellular localization of Arfs and important Arf interactors. While Arfs are shown there are many non-Golgi GEFs, GAPs and effectors not shown. Furthermore, most non-coat effectors are not shown. Note that Arf effectors can vary dramatically between different membranes, and that Golgi-localized GEFs and GAPs are frequently also localized to non-Golgi membranes.

FIGURE 2

Summary of Arf localizations and activities identified by CRISPR/Cas9 knockdown (Pennauer et al., 2021) and superresolution microscopy (Wong-Dillworth et al., 2023), including proposed TGN localization and roles for Arf4 in trafficking to cilia (Deretic et al., 2021).

FIGURE 3

The same Arf/GEF combination can recruit distinct effectors to different membranes. (A) Arf4 can be recruited to cis-Golgi by GBF1, and in turn it recruits the COPI coat. (B) (left) Arf4 recruited to TGN membranes by GBF1 can recruit GGA and the Arf3 exchange factors BIG1/2 to support clathrin vesicle trafficking to endosomes (right) Arf4 can be recruited to TGN by GBF1 where, in distinct nanodomains, it binds rhodopsin and other factors to be trafficked to cilia.

FIGURE 4

Generic model for recruitment of Arf effectors. The effector will typically require binding to the Arf-GTP. However, other interactions must be necessary to maintain specificity, as the same Arf may be present on other membranes. These other interactions could include: direct interaction with the Arf GEF, binding to membrane-specific Rab effectors (e.g., p115 in part A), binding to other membrane-specific proteins such as cargo proteins, or binding to modified membrane lipids such as phosphoinositides.