Negative regulator of MAP3K1/2 signaling. Converts MAP3K2 from its phosphorylated form to its non-phosphorylated form and inhibits autophosphorylation of MAP3K2. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by binding of S100B which releases autoinhibitory N-lobe interactions, enabling ATP to bind and the autophosphorylation of Ser-281. Thr-444 then undergoes calcium-dependent phosphorylation by STK24/MST3. Interactions between phosphorylated Thr-444 and the N-lobe promote additional structural changes that complete the activation of the kinase. Autoinhibition is also released by the binding of MOB1/MOBKL1A and MOB2/HCCA2 to the N-terminal of STK38. Homodimeric S100B binds two molecules of STK38 (PubMed:14661952). Interacts with MOB1 and MOB2 (PubMed:15067004, PubMed:15197186). Interacts with MAP3K1 and MAP3K2 (via the kinase catalytic domain) (PubMed:17906693). Forms a tripartite complex with MOBKL1B and STK3/MST2 (PubMed:18362890). Interacts with MICAL1; leading to inhibit the protein kinase activity by antagonizing activation by MST1/STK4 (By similarity). Ubiquitously expressed with highest levels observed in peripheral blood leukocytes. ISGylated. Phosphorylated by STK3/MST2 and this is enhanced by MOBKL1B. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. Was originally thought to be part of the MLL5-L complex, at least composed of KMT2E, STK38, PPP1CA, PPP1CB, PPP1CC, HCFC1, ACTB and OGT (PubMed:19377461). However, the corresponding article has been retracted (PubMed:24336203).