8.A.109. The Endoplasmic Reticulum Junction-forming protein (Lunapark) Family
Lnp1p, a member of the conserved Lunapark family, plays a role in ER network formation in yeast. Lnp1p binds to the reticulons and Yop1p and resides at ER tubule junctions in both yeast and mammalian cells. In the yeast Saccharomyces cerevisiae, the interaction of Lnp1p with the reticulon protein, Rtn1p, and the localization of Lnp1p to ER junctions are regulated by Sey1p, the yeast orthologue of atlastin. Chen et al. 2012 proposed that Lnp1p and Sey1p act antagonistically to balance polygonal network formation. In support of this proposal, they showed that the collapsed, densely reticulated ER network in lnp1 Δ cells is partially restored when the GTPase activity of Sey1p is abrogated.
Mammalian Lnp1 (mLnp1) affects ER junction mobility and hence network dynamics. Three-way junctions with mLnp1 are less mobile than junctions without mLnp1. Newly formed junctions that acquire mLnp1 remain stable within the ER network, whereas nascent junctions that fail to acquire mLnp1 undergo rapid ring closure. Thus, mLnp1 plays a key role in stabilizing nascent three-way ER junctions (Chen et al. 2015). Moreover, Lnp1 maintains nucleopore stability and plays a role in NPC integrity, separate from its functions in the ER. It may be linked to Ndc1 and Rtn1 interactions (Casey et al. 2015). In plants (A. thaliana), AtLNP1 and AtLNP2 are involved in determining the network morphology of the plant ER, possibly by regulating the formation of ER cisternae (Kriechbaumer et al. 2018).