Promotes cell surface expression of the potassium channel KCND2 (PubMed:12575952). Modulates the activity and gating characteristics of the potassium channel KCND2 (PubMed:12575952, PubMed:16123112, PubMed:19279261, PubMed:19901547). Has no dipeptidyl aminopeptidase activity (By similarity). Homodimer (in vitro) (By similarity). Interacts with KCND2 (PubMed:12575952). Identified in a complex with KCND2 and KCNIP2. Forms an octameric complex composed of four DPP6 subunits bound to the KCND2 tetramer. Detected in brain cortex, hippocampus, thalamus and cerebellum granule cells (at protein level) (PubMed:16123112). Isoform DPPX-S is expressed in brain and some peripheral tissues including kidney, ovary, and testis; in contrast isoform DPPX-L is expressed almost exclusively in brain. N-glycosylated. Belongs to the peptidase S9B family.