Enzyme SPPL3 helps activate our immune system
Already known to cut proteins, the enzyme SPPL3 turns out to have additional properties. In a newly discovered role SPPL3 activates T cells — the immune system's foot soldiers.
Because SPPL3's structure is similar to that of presenilin enzymes, which are associated with Alzheimer's disease, researchers at The Johns Hopkins University believe they could shed more light on presenilin function and give new insight into how our immune system is controlled.
A summary of these findings from Johns Hopkins University are published in the Dec. 22, 2014 journal of Molecular and Cellular Biology.
"No one could have predicted that SSPL3 was involved in T cell activation," says Joel Pomerantz, Ph.D., an associate professor of biological chemistry at The Johns Hopkins University. "It walks like a duck and quacks like a duck, but its duck-like abilities don't come into play here."
T cells kill invading cells and activate other immune cells. When a foreign protein binds to a receptor on the outside of a T cell, a signal relay begins. That signal calls the NFAT protein to move into the nucleus and turn on a number of genes to get T cells ready for battle. Some of what happens in between is known, but Pomerantz wanted to know the entire process.
The scientists wanted to increase NFAT's response — and found SPPL3. The enzyme proved essential to activating NFAT. SPPL3 had never been implicated in immune system function before and more tests confirmed that SPPL3 was in the sequence leading to NFAT's activation.
SPPL3 lives in the membrane of
the endoplasmic reticulum (ER),
a ruffled membrane inside the
cell which helps process new
proteins. The ER is also where
interactions between STIM1 and
Orai1, components of the NFAT
signal relay system, are found.
SPPL3 seems to accomplish NFAT signalling without using its enzymatic (or protein-cutting) ability. It also stimulates the release of calcium from the ER which promotes signaling.
Says Pomerantz: "SPPL3 is a relatively uncharacterized protein that had never before been implicated in immune system function. It opens up a whole new set of scientific questions."
Pomerantz thinks that SPPL3 could be used as a drug target to either enhance the activation of T cells in immunodeficient individuals or to suppress it in those with overactive immune systems.
He plans to study the ability of SPPL3 to modify the influx of calcium into the cell and the release of calcium from the ER, as calcium is integral to the function of many cell signaling networks.
The signal peptide peptidase (SPP)-related intramembrane aspartyl proteases are a homologous group of polytopic membrane proteins, some of which function in innate or adaptive immunity by cleaving proteins involved in antigen presentation or intracellular signaling. Signal peptide peptidase-like 3 (SPPL3) is a poorly characterized endoplasmic reticulum (ER)-localized member of this family, with no validated cellular substrates. We report here the isolation of SPPL3 in a screen for activators of NFAT, a transcription factor that controls lymphocyte development and function. We find that SPPL3 is required downstream of T cell receptor engagement for maximal Ca2+ influx and NFAT activation. Surprisingly, the proteolytic activity of SPPL3 is not required for its role in this pathway. SPPL3 enhances the signal-induced association of stromal interaction molecule 1 (STIM1) and Orai1 and is even required for the full activity of constitutively active STIM1 variants that bind Orai1 independently of ER Ca2+ release. SPPL3 associates with STIM1 through at least two independent domains, the transmembrane region and the CRAC activation domain (CAD), and can promote the association of the STIM1 CAD with Orai1. Our results assign a function in lymphocyte signaling to SPPL3 and highlight the emerging importance of nonproteolytic functions for members of the intramembrane aspartyl protease family.
Other authors of the report include Stefanie Makowski and Zhaoquan Wang of the Johns Hopkins University School of Medicine.
This work was supported by grants from the National Institute of Allergy and Infectious Diseases (PO1AI072677), the Johns Hopkins University Institute for Cell Engineering and from Dr. Richard and Mavis Fowler and The Foundation for Advanced Research in the Medical Sciences, Inc.
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