The gene in oligodendrocyte lineage (golli) is responsible for a number of intrinsically-disordered proteins including the understudied golli protein BG21 and the 21.5-kDa isoform of the myelin-basic-protein (MBP-21.5). The golli-interacting-protein (GIP) was discovered in a search for potential BG21 interacting-partners. It was shown to interact with BG21, although the inability to produce a full-length version of GIP led to inconclusive results. GIP is an acidic phosphatase belonging to the family of RNA-polymerase-2 C-terminal-phosphatases. Here, a novel protocol for the purification of a functional full-length GIP has been developed with a yield of 4-mg/L culture with 85% purity. An enzymological approach was used to study the GIP-BG21 and GIP-MBP-21.5 interactions. No GIP-MBP-21.5 interactions were found. The phosphorylation state of BG21 was important for the GIP-BG21 interactions. BG21 enhanced GIP (Ka=30 μM) whereas Protein-Kinase-C-phosphorylated BG21 inhibited GIP (Ki=3.5 μM, Ki'=8.0 μM), suggesting a potential role of BG21 and GIP in transcription regulation.