Identification of a Novel Human LAP1 Isoform That Is Regulated by Protein Phosphorylation

Abstract

Lamina associated polypeptide 1 (LAP1) is an integral protein of the inner nuclear membrane that is ubiquitously expressed. LAP1 binds to lamins and chromatin, probably contributing to the maintenance of the nuclear envelope architecture. Moreover, LAP1 also interacts with torsinA and emerin, proteins involved in DYT1 dystonia and X-linked Emery-Dreifuss muscular dystrophy disorder, respectively. Given its relevance to human pathological conditions, it is important to better understand the functional diversity of LAP1 proteins. In rat, the LAP1 gene (TOR1AIP1) undergoes alternative splicing to originate three LAP1 isoforms (LAP1A, B and C). However, it remains unclear if the same occurs with the human TOR1AIP1 gene, since only the LAP1B isoform had thus far been identified in human cells. In silico analysis suggested that, across different species, potential new LAP1 isoforms could be generated by alternative splicing. Using shRNA to induce LAP1 knockdown and HPLC-mass spectrometry analysis the presence of two isoforms in human cells was described and validated: LAP1B and LAP1C; the latter is putatively N-terminal truncated. LAP1B and LAP1C expression profiles appear to be dependent on the specific tissues analyzed and in cultured cells LAP1C was the major isoform detected. Moreover, LAP1B and LAP1C expression increased during neuronal maturation, suggesting that LAP1 is relevant in this process. Both isoforms were found to be post-translationally modified by phosphorylation and methionine oxidation and two LAP1B/LAP1C residues were shown to be dephosphorylated by PP1. This study permitted the identification of the novel human LAP1C isoform and partially unraveled the molecular basis of LAP1 regulation.

Genetic mutations strengthen functional association of LAP1 with DYT1 dystonia and muscular dystrophy

“…Interestingly, in human brain, LAP1B is more abundant than LAP1C [26]. LAP1B's higher expression levels appear to be related to highly differentiated cells [26]. This is exemplified during differentiation of rat cortical primary cultures.…”

“…Protein levels are similar in ovary, testis and pancreas. Interestingly, in human brain, LAP1B is more abundant than LAP1C [26]. LAP1B's higher expression levels appear to be related to highly differentiated cells [26].…”

“…Recently another human LAP1 isoform was identified; LAP1C was shown to be present in the human cell line SH-SY5Y and also in human tissue samples [26]. Comparatively, LAP1C has a shorter Nterminal domain ( Fig.…”

“…Subsequently the same isoforms were described for mouse. In the latter there are at least three RefSeq records corresponding to three different mouse LAP1 transcripts: transcript 1 (NM_001160018) the longest transcript, transcript 2 (NM_144791) is shorter than transcript 1 and transcript 3 (NM_001160019) which represents the smallest transcript and lacks the N-terminus [26]. However, in human, not all three isoforms were originally identified.…”

Protein phosphatase 1 is a key player in nuclear events

“…Nuclear envelope (NE) proteins have a highly relevant role in this process, namely emerin and LAP1 [81,82], the latter protein was recently identified as a novel PP1 substrate. In fact it was established that PP1 is able to dephosphorylate two LAP1 residues: Ser306 and Ser310 [83,84].…”