Synaptojanin (SYNJ) 1 and 2 are inositol-5-phosphatases that sequentially convert PI(4,5)P2 to PI(4)P and PI (Cremona et al 1999; reviewed in Billcliff and Lowe, 2014). Conversion of PI(4,5)P2 to PI(4)P and PI accompanies maturation of the clathrin-coated pit, and consistent with this, SYNJ proteins are recruited to the clathrin-coated pit through interactions with a number of endocytic proteins including ITSNs, EPS15, PACSIN proteins and endophilins, as well as with clathrin and AP-2 (Haffner et al, 1997; Cestra et al, 1999; Maire et al, 2004; Schuske et al, 2003; Verstreken et al, 2003; Modregger et al, 2000; Perera et al 2006; Milosevic et al, 2011; reviewed in Dittman and Ryan, 2009). SYJN1 exists in two isoforms, a longer 170 kDA isoform and a shorter 145 kDA isoform, with slightly different roles. The recruitment and activity of SYNJ1-145 appears to largely coincide with that of dynamin at later stages of vesicle formation, while the SYNJ1-170 isoform also plays earlier roles in stabilizing the growing clathrin-coated vesicle (Perera et al, 2006; Taylor et al, 2011; Antonescu et al, 2011). SYNJ-mediated hydrolysis of PI(4,5)P2 to PI(4)P is most efficient on highly curved, endophilin-coated tubules of the vesicle neck and contributes to dynamin-mediated membrane scission (Chang-Ileto et al, 2011; reviewed in Daumke et al, 2014; McMahon and Boucrot, 2011).
In addition to SYNJ1 and 2, other inositol-5-phosphatases are also recruited to the CCP at the time of scission. These include OCRL, which is recruited through interaction with clathrin as well as the RAB5 interactor APPL1 (Erdmann et al, 2007; Mao et al, 2009; Taylor et al, 2011; Nandez et al, 2014).