The function of TTLL1 (TTL family tubulin polyglutamylase complex subunit L1, ENSG00000100271) is as follows. Catalytic subunit of a polyglutamylase complex which modifies tubulin, generating side chains of glutamate on the gamma-carboxyl group of specific glutamate residues within the C-terminal tail of tubulin (PubMed:34782749). Probably involved in the side-chain elongation step of the polyglutamylation reaction rather than the initiation step. Modifies both alpha- and beta-tubulins with a preference for the alpha-tail. Unlike most polyglutamylases of the tubulin--tyrosine ligase family, only displays a catalytic activity when in complex with other proteins as it is most likely lacking domains important for autonomous activity. Part of the neuronal tubulin polyglutamylase complex. Mediates cilia and flagella polyglutamylation which is essential for their biogenesis and motility. Involved in respiratory motile cilia function through the regulation of beating asymmetry. Essential for sperm flagella biogenesis, motility and male fertility. Involved in KLF4 glutamylation which impedes its ubiquitination, thereby leading to somatic cell reprogramming, pluripotency maintenance and embryogenesis. .