Lamins, the nuclear intermediate filaments, are important regulators of nuclear structural integrity as well as nuclear functional processes such as DNA transcription, replication and repair, and epigenetic regulations. A portion of phosphorylated lamin A/C localizes to the nuclear interior in interphase, forming a lamin A/C pool with specific properties and distinct functions. Nucleoplasmic lamin A/C molecular functions are mainly dependent on its binding partners; therefore, revealing new interactions could give us new clues on the lamin A/C mechanism of action. In the present study, we show that lamin A/C interacts with nuclear phosphoinositides (PIPs), and with nuclear myosin I (NM1). Both NM1 and nuclear PIPs have been previously reported as important regulators of gene expression and DNA damage/repair. Furthermore, phosphorylated lamin A/C forms a complex with NM1 in a phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2)-dependent manner in the nuclear interior. Taken together, our study reveals a previously unidentified interaction between phosphorylated lamin A/C, NM1, and PI(4,5)P2 and suggests new possible ways of nucleoplasmic lamin A/C regulation, function, and importance for the formation of functional nuclear microdomains.

Epstein-Barr virus (EBV) is a human herpesvirus that causes a variety of hematological malignancies, in particular B-, T- and NK-cell lymphomas. Latency proteins of the virus contribute to lymphomagenesis by activating the transcription factor NF-κB, however, the molecular mechanism underlying EBV-driven NF-κB activation remains incompletely understood. Here we show that EBV latent membrane proteins LMP1 and LMP2A or LMP2B synergize to activate the protease MALT1, a key driver of NF-κB-mediated lymphocyte proliferation. EBV-positive B-cell lines co-expressing LMP1 and LMP2 proteins exhibited constitutive MALT1 activity, and treatment with a MALT1 inhibitor or silencing of latent membrane protein-1 (LMP1) or LMP2 impaired MALT1-dependent substrate cleavage. LMP1 physically interacted with the MALT1-binding partner BCL10, while LMP2A and LMP2B interacted with MALT1. LMP1 also colocalized with LMP2 in dotted structures, suggesting that LMP1 and LMP2 synergize via the coordinated formation of LMP-BCL10-MALT1 (LBM) complexes. Finally, MALT1 protease inhibition led to reduced expression of a set of NF-κB-driven genes, and reduced tumor growth in an EBV-positive lymphoma xenograft model. These findings establish a key role for the protease MALT1 in LMP1/2-mediated, NF-κB-driven cellular transformation and provide a rationale for inhibiting MALT1 to treat EBV-positive B-cell lymphomas that co-express LMP1 and LMP2.

Tumor suppressors represent one of the first lines of defense against malignant transformation, and their inactivation in cells leads to the onset of tumorigenesis. Cancer cells employ various mechanisms to inactivate cellular tumor suppressors, including epigenetic silencing and mutagenesis. Less understood are the mechanisms by which cancer cells inactivate tumor suppressors post-translationally. Here, we uncovered a previously undescribed post-translational mechanism by which cancer cells inactivate the potent mitochondrial tumor suppressor LACTB in breast cancers. We discovered that the substrate of LACTB can be LACTB itself; LACTB possesses autoproteolytic activity, which is important for modulating its tumor suppressor activity. We demonstrate that cancer cells exploit this feature of LACTB to induce LACTB’s self-degradation. This is mechanistically realized through upregulation of mitochondrial MRPS34 protein, which, through interaction with LACTB, is a positive regulator of LACTB’s autoproteolytic activity and a negative regulator of LACTB. This study provides insights into how cancer cells fine-tune the expression and activity of tumor suppressors to promote tumorigenesis.