Spastic Paraplegia 21 (SPG21)

SPG21 was identified by a two-hybrid screen using CD4 as the bait. It binds to the hydrophobic C-terminal amino acids of CD4 which are involved in repression of T cell activation. The interaction with CD4 is mediated by the noncatalytic alpha/beta hydrolase fold domain of this protein. It is thus proposed that this gene product modulates the stimulatory activity of CD4. At least three different transcript variants encoding two different isoforms have been found for this gene.The protein contains a cluster of 4 aspartic acids near the N terminus and a structural element conserved in alpha/beta fold bacterial hydrolases. It does not have a localization signal, leader sequence, or transmembrane helix. Northern blot analysis detected a 2.1-kb ACP33 transcript expressed at comparable levels in all tissues examined.