Both PY-NLS and Ser1027 residue phosphorylation in ULK2 confer a unique role in autophagy.

P794A (⁷⁷⁴gpgfgssppGaeaapslRyvPY⁷⁹⁵ changed to ⁷⁷⁴gpGfgssppgaeaapslRyvAY⁷⁹⁵) or P242A (²²⁰qdlrmfyeKnRnotslmpSipRetsPY²⁴³ changed to ²²⁰qdlrmfyekKnRslmpSipRetsAY²⁴³) mutants showed an increased autophagic activity, but less serine phosphorylation. Membrane attachment and interaction with Atg13-focal adhesion kinase family-interacting protein 200 (FIP200) appears to mask the ULK2 PY NLS motif (Left side). The protein complex is likely dissociated by phosphorylation by the protein kinase (such as AMPK, mTOR1, PKA, or ULK2), consequently making the PY-NLS motif in ULK2 accessible by Kapβ2 (Right side), even though the phosphorylation sites on Atg13-FIP200 by the protein kinases are not completely characterized. Due to the fact that the PY-NLS mutant of ULK2 and WT ULK1 (“X” indicates blockade of Kapβ2 binding) are not transported to the nucleus, these two seem to be more active in autophagy than WT ULK2. The ULK2 S1027A mutant (dephosphorylated analogy form) can easily bind Atg13-FIP200 to promote cell autophagy and apoptosis. These protein associations result in its PY-NLS motif hiding through steric inhibition, resulting in the blockade of its nuclear localization by Kapβ2 (Left side). Meanwhile, the PY-NLS motif of the ULK2 S1027D mutant (PKA phosphorylated analogy form), which is free from Atg13-FIP200 association, is exposed to Kapβ2, and the mutant protein can be imported into the nuclei (Right side). The freed ULK2 S1027D mutant can induce neither cell autophagy nor apoptosis. Thus, the phosphorylation on the Ser1027 residue (RRlSA) of ULK2 by PKA seems to be a major regulatory event in its autophagic functions (see the main body text for more detail).