GTPase tethering membranes through formation of trans- homooligomer and mediating homotypic fusion of endoplasmic reticulum membranes. Functions in endoplasmic reticulum tubular network biogenesis. May also regulate Golgi biogenesis. May regulate axonal development. Defects in ATL1 are the cause of spastic paraplegia autosomal dominant type 3 (SPG3); also known as Strumpell-Lorrain syndrome. Spastic paraplegia is a degenerative spinal cord disorder characterized by a slow, gradual, progressive weakness and spasticity of the lower limbs. Defects in ATL1 are the cause of hereditary sensory neuropathy type 1D (HSN1D). HSN1D is a disease characterized by adult-onset distal axonal sensory neuropathy leading to mutilating ulcerations as well as hyporeflexia. Some patients may show features suggesting upper neuron involvement. Belongs to the GBP family. Atlastin subfamily. Note: This description may include information from UniProtKB.