Catalyzes the first and rate-limiting reaction of the four reactions that constitute the long-chain fatty acids elongation cycle. This endoplasmic reticulum-bound enzymatic process allows the addition of 2 carbons to the chain of long- and very long-chain fatty acids (VLCFAs) per cycle. a very-long-chain acyl-CoA + H(+) + malonyl-CoA = a very-long-chain 3-oxoacyl-CoA + CO2 + CoA RNAi-mediated knockdown leads to lethality before eclosion (PubMed:29739804). RNAi-mediated knockdown in neurons leads to neurodegeneration in the central nervous system including degeneration of retina, defects in the fenestrated basement membrane and ommatidial disarray (PubMed:29739804). Effects are probably due to altered levels of very-long chain fatty acids (VLCFAs) (PubMed:29739804). In contrast, glial-specific RNAi-mediated knockdown results in no defect (PubMed:29739804). Belongs to the ELO family.