Sp140 is a multi-SUMO-1 target and its PHD finger promotes SUMOylation of the adjacent Bromodomain

BACKGROUND

Human Sp140 protein is a leukocyte-specific member of the speckled protein (Sp) family (Sp100, Sp110, Sp140, Sp140L), a class of multi-domain nuclear proteins involved in intrinsic immunity and transcriptional regulation. Sp140 regulates macrophage transcriptional program and is implicated in several haematologic malignancies. Little is known about Sp140 structural domains and its post-translational modifications.

METHODS

We used mass spectrometry and biochemical experiments to investigate endogenous Sp140 SUMOylation in Burkitt's Lymphoma cells and Sp140 SUMOylation sites in HEK293T cells, FLAG-Sp140 transfected and His₆-SUMO-1^T95K infected. NMR spectroscopy and in vitro SUMOylation reactions were applied to investigate the role of Sp140 PHD finger in the SUMOylation of the adjacent BRD.

RESULTS

Endogenous Sp140 is a SUMO-1 target, whereby FLAG-Sp140 harbors at least 13 SUMOylation sites distributed along the protein sequence, including the BRD. NMR experiments prove direct binding of the SUMO E2 ligase Ubc9 and SUMO-1 to PHD-BRD_Sp140. In vitro SUMOylation reactions show that the PHD_Sp140 behaves as SUMO E3 ligase, assisting intramolecular SUMOylation of the adjacent BRD.

CONCLUSIONS

Sp140 is multi-SUMOylated and its PHD finger works as versatile protein-protein interaction platform promoting intramolecular SUMOylation of the adjacent BRD. Thus, combinatorial association of Sp140 chromatin binding domains generates a multifaceted interaction scaffold, whose function goes beyond the canonical histone recognition.

GENERAL SIGNIFICANCE

The addition of Sp140 to the increasing lists of multi-SUMOylated proteins opens new perspectives for molecular studies on Sp140 transcriptional activity, where SUMOylation could represent a regulatory route and a docking surface for the recruitment and assembly of leukocyte-specific transcription regulators.

The biophysical and biochemical properties of the autoimmune regulator (AIRE) protein

AIRE (for autoimmune regulator) is a multidomain protein that performs a fundamental function in the thymus and possibly in the secondary lymphoid organs: the regulation, especially in the sense of activation, of the process of gene transcription in cell lines deputed to the presentation of self-antigens to the maturing T lymphocytes. The apoptosis of the elements bearing T-cell receptors with critical affinity for the exhibited self-antigens prevents the escape of autoreactive clones and represents a simple and efficient mechanism of deletional self-tolerance. However, AIRE action relies on an articulated complex of biophysical and biochemical properties, in most cases attributable to single subspecialized domains. Here a thorough review of the matter is presented, with a privileged look at the pathogenic changes of AIRE that interfere with such properties and lead to the impairment in its chief function.