ARL8A

ARL8A is an Arf-like GTPase that resides on lysosomes and acts as a positive regulator of microtubule-dependent lysosomal positioning (PMID:16537643). Distinct from canonical Arf/Arl proteins, ARL8A and its paralog ARL8B lack N-terminal myristoylation and instead rely on N-terminal acetylation for lysosomal membrane targeting (PMID:16537643). On the lysosomal surface, ARL8A mediates recruitment of plus-end-directed kinesin motors, including kinesin-1 and kinesin-3 (KIF1Bβ), driving lysosome movement toward the cell periphery; overexpression shifts lysosomes peripherally and increases the frequency of bidirectional movement (PMID:16537643, PMID:34878110). Independent of its lysosomal role, ARL8A (Gie2) binds tubulin directly, localizes to microtubules at the spindle mid-zone in late mitosis, and is required for normal chromosome segregation (PMID:15331635). Beyond these roles, no further mechanistic detail—including GTPase regulatory cycle, effector structures, or guanine-nucleotide exchange factors—has been characterized in the available corpus.

1. 2004 Medium

   Established the first cellular role for ARL8A by linking it to mitotic fidelity, showing it is not merely a housekeeping GTPase but acts on the mitotic spindle.

   Evidence Dominant-negative overexpression in mammalian cells and RNAi knockdown in Drosophila S2 cells with tubulin co-sedimentation and immunofluorescence

   PMID:15331635

   Open questions at the time

   • Tubulin interaction shown by co-sedimentation only, not confirmed by reciprocal Co-IP or structural data
   • Single lab
   • Relationship between mitotic role and lysosomal role not resolved
2. 2006 High

   Defined ARL8A as a lysosome-resident GTPase using a non-canonical N-terminal acetylation signal for membrane targeting, and as a positive regulator of microtubule-dependent lysosomal motility.

   Evidence Live cell imaging, subcellular fractionation, N-terminal acetylation biochemistry, and acetylation-site mutagenesis in mammalian and Drosophila cells

   PMID:16537643

   Open questions at the time

   • Identity of the motors and adaptors mediating peripheral redistribution not defined at this stage
   • GTPase cycle and regulators (GEF/GAP) not identified
   • Mechanism reconciling lysosomal residency with spindle localization not addressed
3. 2022 Medium

   Resolved the motor-recruitment mechanism underlying ARL8A-driven lysosomal motility, showing it recruits kinesin-1 and kinesin-3 (KIF1Bβ) to lysosomes.

   Evidence Genetic epistasis in infected vs. uninfected cells, Arl8a/b depletion, co-immunoprecipitation, and fluorescence imaging of lysosomal kinesin recruitment

   PMID:34878110

   Open questions at the time

   • Single lab; Co-IP-based interaction not structurally characterized
   • Functional contributions of ARL8A versus ARL8B not separated
   • Adaptor proteins bridging ARL8A to kinesins not detailed in this finding

• How ARL8A coordinates its two distinct roles—lysosomal motor recruitment and mitotic spindle function—and how its GTPase nucleotide cycle is regulated remain unresolved.
• No GEF or GAP for ARL8A identified
• No structural model of ARL8A-effector or ARL8A-tubulin interactions
• Division of labor between ARL8A and ARL8B paralogs uncharacterized