The long signal peptide isoform and its alternative processing direct the intracellular trafficking of interleukin-15

Two isoforms of interleukin (IL)-15 exist: one with a short and another wit h a long signal peptide (LSP). Experiments using combinations of the LSP an d mature proteins IL-2, IL-15, and green fluorescent protein revealed compl ex pathways of intracellular trafficking. In one pathway, the LSP was unpro cessed, and IL-15 was not glycosylated, remained in the cytoplasm, and was degraded. The second trafficking pathway involved endoplasmic reticulum ent ry, N-linked glycosylation, and alternative partial LSP processing. The thi rd pathway involved endoplasmic reticulum entry, followed by glycosylation, complete processing, and ultimately secretion. The complex intracellular t rafficking patterns of LSP-IL-15 with its impediments to secretion as well as impediments to translation may be required due to the potency of IL-15 a s an inflammatory cytokine. In terms of a more positive role, we propose th at intracellular infection may relieve the burdens on translation and intra cellular trafficking to yield effective IL-15 expression.