ABSTRACT Neurofibromin (Nf1) is a large multidomain protein encoded by the tumour-suppressor gene NF1 . NF1 is mutated in a frequently occurring genetic disease, neurofibromatosis type I, and in various cancers. The best described function of Nf1 is its Ras-GTPase activity, carried out by its GAP-related domain (GRD). SecPH, another structurally well-characterized domain of Nf1, is immediately adjacent to the GRD and interacts with lipids and proteins, thus connecting Nf1 to diverse signalling pathways. Here, we demonstrate, for the first time, that Nf1 and SecPH are substrates of the SUMO pathway. We identified a well-defined SUMOylation profile of SecPH and a main SUMOylation event on Lys1731 that appears to play a role in Ras-GAP activity. Our data allowed us to characterize a new set of pathogenic Nf1 missense mutants that exhibits a disrupted SUMOylation profile that may correlate with their unfolding. Accordingly, Lys1731 SUMOylation is mediated by a noncanonical structural motif, therefore allowing a read-out of SecPH conformation and folding status.