Progesterone (P4) receptor membrane component (PGRMC)1 is detected as a 22-kDa band as well as higher molecular mass bands (>50 kDa) in spontaneously immortalized granulosa cells. That these higher molecular mass bands represent PGRMC1 is supported by the findings that they are not detected when either the primary antibody is omitted or the PGRMC1 antibody is preabsorbed with recombinant PGRMC1. Some but not most of the higher molecular mass bands are due to oligomerization. At least one of the higher molecular mass bands is sumoylated, because PGRMC1 coimmunoprecipitates with small ubiquitin-like modifier protein-1. Moreover, in situ proximity ligation assays reveal a direct interaction between PGRMC1 and small ubiquitin-like modifier protein-1. This interaction is increased by P4. Finally, the higher molecular mass forms of PGRMC1 localize to the nucleus. An analysis of transcription factor activity demonstrates that P4 suppresses T-cell factor/lymphoid enhancer factor (Tcf/Lef) activity through a PGRMC1-dependent mechanism, because treatment with PGRMC1 small interfering RNA depletes PGRMC1 levels and attenuates P4's effects on Tcf/Lef activity. In addition, transfection of a PGRMC1-Flag fusion protein enhances basal Tcf/Lef activity, which is suppressed by P4 treatment. Conversely, transfection of a PGRMC1-Flag protein in which all the sumoylation sites are mutated increases basal Tcf/Lef activity but attenuates P4's ability to suppress Tcf/Lef activity. Therefore, the ability to suppress Tcf/Lef activity is likely an essential part of the mechanism through which P4 activation of PGRMC1 regulates the gene cascades that control granulosa cell function with this action being dependent in part on the sumoylation status of PGRMC1.