1. ¹Department of Microbiology and Brain Korea 21 Project of Medical Sciences, Yonsei University College of Medicine, Yonsei University, Seoul, Korea
2. ²Department of Internal Medicine, Yonsei University College of Medicine, Seoul, Korea
3. ³Department of Chemistry, Inha University, Incheon, Korea

Correspondence: Dr J Kim, Department of Microbiology and Institute for Immunology and Immunological Diseases, Yonsei University College of Medicine, 134 Shinchon-dong, Seodaemoon-gu, Seoul 120-752, Korea. E-mail: jkim63@yumc.yonsei.ac.kr

Received 24 May 2007; Revised 17 December 2007; Accepted 20 January 2008; Published online 26 February 2008.

Abstract

Activation-induced upregulation of inhibitory killer Ig-like receptor (KIR) is regulated by protein kinase Cs (PKCs). Conventional PKCs increase KIR expression on the post-transcriptional level by increasing the recycling of surface molecules and endoplasmic reticulum (ER)–Golgi processing. PKC plays a role in the secretion of cytoplasmic KIR through lytic granules. In this study, we identified amino acid sequence motifs associated with PKC-mediated KIR membrane trafficking by systematic mutagenesis. Mutations of Y³⁹⁸ and HLWC³⁶⁴ completely inhibited the PMA-induced increase of KIR molecules at surface as well as total protein levels, indicating that these are associated with ER–Golgi processing and sorting to plasma membrane through lytic granules. Mutations of Y-based motif, including Y³⁹⁸, acidic region (PE³⁹⁴), dileucine motif-like region (IL⁴²³) and PKC-phosphorylatable S⁴¹⁵ caused a blockade of surface KIR endocytosis after PKC stimulation. Mutation of T¹⁴⁵ caused an accumulation of mutant proteins in late endosomes and lysosomes after PKC activation, suggesting that T¹⁴⁵ might be related to the recovery of endocytosed KIR to the surface membrane. We also demonstrated that PKCs could directly phosphorylate the KIR cytoplasmic tail by means of western blot and in vitro kinase assay, implying that phosphorylation status of KIR cytoplasmic tail can direct the fate of surface KIR molecules. Taken together, various sequence motifs are implicated in the PKC-mediated post-transcriptional upregulation of KIR, and each of these motifs work in different steps after PKC activation.