Unique and Shared Functions of Nuclear Lamina LEM Domain Proteins in Drosophila

Abstract

The nuclear lamina is an extensive protein network that contributes to nuclear structure and function. LEM domain (LAP2, emerin, MAN1 domain, LEM-D) proteins are components of the nuclear lamina, identified by a shared 45-amino-acid motif that binds Barrier-to-autointegration factor (BAF), a chromatin-interacting protein. Drosophila melanogaster has three nuclear lamina LEM-D proteins, named Otefin (Ote), Bocksbeutel (Bocks), and dMAN1. Although these LEM-D proteins are globally expressed, loss of either Ote or dMAN1 causes tissue-specific defects in adult flies that differ from each other. The reason for such distinct tissue-restricted defects is unknown. Here, we generated null alleles of bocks, finding that loss of Bocks causes no overt adult phenotypes. Next, we defined phenotypes associated with lem-d double mutants. Although the absence of individual LEM-D proteins does not affect viability, loss of any two proteins causes lethality. Mutant phenotypes displayed by lem-d double mutants differ from baf mutants, suggesting that BAF function is retained in animals with a single nuclear lamina LEM-D protein. Interestingly, lem-d double mutants displayed distinct developmental and cellular mutant phenotypes, suggesting that Drosophila LEM-D proteins have developmental functions that are differentially shared with other LEM-D family members. This conclusion is supported by studies showing that ectopically produced LEM-D proteins have distinct capacities to rescue the tissue-specific phenotypes found in single lem-d mutants.Our findings predict that cell-specific mutant phenotypes caused by loss of LEM-D proteins reflect both the constellation of LEM-D proteins within the nuclear lamina and the capacity of functional compensation of the remaining LEM-D proteins. T HE nuclear lamina is an extensive protein network underlying the nuclear envelope. This network is composed of the nucleus-specific intermediate filament proteins, the Aand B-type lamins, which form a structural platform for association of .200 proteins (Schirmer et al. 2003;Korfali et al. 2010;Malik et al. 2010). LEM domain (LAP2, emerin, MAN1 domain, LEM-D) proteins represent one family of lamin interacting proteins. This family shares an 45-residue bihelical domain that was first identified in LAP2, emerin, and MAN1 (Lin et al. 2000;Mansharamani and Wilson 2005;Wagner and Krohne 2007). LEM-D proteins interact with the small, conserved protein called Barrier-to-autointegration factor (BAF), a protein that binds double-strand DNA and histones (Zheng et al. 2000;Cai et al. 2001;Laguri et al. 2001;Furukawa et al. 2003;Liu et al. 2003;Montes de Oca et al. 2005). Through interactions with BAF, LEM-D proteins connect interphase chromosomes to the nuclear lamina, thereby contributing to global nuclear organization.Metazoan genomes encode several LEM-D proteins (Lee and Wilson 2004;Berk et al. 2013). Most show enriched localization within the nuclear lamina, wherein the LEM-D proteins direct shared protein associations. For example, emerin and MAN1 in...

Networking in the nucleus: a spotlight on LEM-domain proteins

“…For example, LAP2α–deficient and emerin-deficient muscles both have higher numbers of satellite stem cells and altered myoblast differentiation associated with mis-regulation of the Rb1/E2F pathway [47, 54]. Second, loss of two LEM-D proteins causes phenotypes significantly more severe than loss of single LEM-D proteins [44, 45, 52, 60, 69]. Functional redundancy is perhaps expected between LEM-D proteins in the same group, but not between LEM-D proteins in different groups due to their limited homology outside of the LEM-D.…”

“…Functional redundancy is perhaps expected between LEM-D proteins in the same group, but not between LEM-D proteins in different groups due to their limited homology outside of the LEM-D. However functional overlap between group I and group II LEM-D proteins was first described in worms [44, 45] and later found in other organisms [43, 52, 60, 69]. A reasonable explanation for this overlap is the additive loss of the common interaction with BAF.…”

“…Yet, mounting evidence suggests that redundancy extends beyond BAF association. Genetic studies in flies showed that loss of any two of the three nuclear lamina LEM-D proteins is lethal; the timing of death varies depends upon which specific LEM-D proteins were lost [69]. Furthermore, the mutant phenotypes of lem-d double mutants differed from those of baf mutants, emphasizing that shared functions of the fly LEM-D proteins are not limited to BAF recruitment.…”

“…On one hand, these findings suggest that loss-of-function phenotypes associated with defects in a single LEM-D protein may be tissue-restricted because other members of the LEM-D family are present at sufficient levels in other tissues to maintain critical functions of the nuclear lamina. Nevertheless, defects caused by loss of individual LEM-D proteins also suggest certain functions are unique and cannot be compensated by other family members [43, 52, 60, 69]. Thus, only some LEM-D protein functions are shared with other family members.…”

Drosophila male and female germline stem cell niches require the nuclear lamina protein Otefin

“…In many cases, LEM-D proteins interact with the same regulatory proteins, possibly due to conformational plasticity imparted by the presence of unstructured regions within these proteins (Berk et al, 2014). Such sharing of protein partners provides a framework for understanding why individual loss of these globally expressed LEM-D proteins causes limited developmental defects (Barton et al, 2014; Huber et al, 2009; Liu et al, 2003; Reil and Dabauvalle, 2013). Indeed, loss of single LEM-D proteins causes tissue-restricted human diseases, including bone density disorders, cardiomyopathies and muscular dystrophies (Worman et al, 2010).…”