ALS2 is the gene mutated in a recessive juvenile form of amyotrophic lateral sclerosis (ALS2). ALS2 encodes a large protein termed alsin, which contains a number of predicted cell signaling and protein trafficking sequence motifs. To gain insight into the overall function of alsin and to begin to evaluate its role in motor neuron maintenance, we examined the subcellular localization of alsin and the biochemical activities associated with its individual subdomains. We found that the Vps9p domain of alsin has Rab5 guanine nucleotide exchange activity. In addition, alsin interacted specifically with and acted as a guanine nucleotide exchange factor for Rac1. Immunofluorescence and fractionation experiments in both fibroblasts and neurons revealed that alsin is a cytosolic protein, with a significant portion associated with small, punctate membrane structures. Many of these membrane structures also contained Rab5 or Rac1. Upon overexpression of full-length alsin, the overexpressed material was largely cytosolic, indicating that the association with membrane structures could be saturated. We also found that alsin was present in membrane ruffles and lamellipodia. These data suggest that alsin is involved in membrane transport events, potentially linking endocytic processes and actin cytoskeleton remodeling.Amyotrophic lateral sclerosis (ALS) 1 is a heterogeneous neurological disorder characterized by progressive degeneration of motor neurons, usually causing death as a result of respiratory paralysis (1, 2). Although mostly sporadic in nature, a genetic link has been established in 5-10% of ALS cases (3). Chromosomal mapping studies have identified six independent loci associated with the familial forms of ALS (Refs. 4 -10; reviewed in Refs. 11 and 12). Molecular genetic analysis identified two genes that, when mutated, lead to ALS. The first discovered was the gene coding for Cu-Zn superoxide dismutase 1 (SOD1) (10). Initially, mutations in SOD1 were thought to result in defective free radical scavenger activity. However, it is now generally accepted that the alterations in SOD1 that lead to ALS are the result of an unknown, but toxic gain-of-function. The second gene identified is mutated in a juvenile form of ALS, ALS2 (13,14). Mutations in this gene lead to a rare recessive form of ALS that presents early in life and progresses much more slowly than the classical form (6, 15). Two small deletions in ALS2 were originally associated with the disease (13,14). Each is expected to severely truncate the predicted protein product of ALS2. In addition, mutations in ALS2 have recently been associated with two other neurodegenerative disorders, juvenile-onset primary lateral sclerosis (14) and infantile-onset hereditary spastic paralegia (16 -18). Like the original mutations identified, these mutations are predicted to generate prematurely truncated forms of the protein product.The protein encoded by ALS2, alsin, is predicted to contain numerous domains implicating roles in cell signaling, membrane localization, and protein...