The E3 ubiquitin ligase c-IAP1 regulates PCSK9-mediated LDLR degradation: Linking the TNF-α pathway to cholesterol uptake

Proprotein convertase subtilisin/kexin type 9 (PCSK9), in addition to LDLR (low-density lipoprotein receptor) and APOB (apolipoprotein B), is one of three loci implicated in autosomal dominant hypercholesterolaemia (ADH)^1^. A number of PCSK9 gain-of-function mutations and loss-of-function mutations have been identified from families afflicted with ADH with hypercholesterolaemia or hypocholesterolaemia, respectively^1-4^. In humans, the main function of PCSK9 appears to be the post-transcriptional regulation of the number of cell-surface LDL receptors^5-7^. To date, only LDLR and its closest family members VLDLR and ApoER2 have been shown to bind with PCSK9^8,9^. To find new binding partners for PCSK9, we used a shotgun proteomic method to analyse the protein complex pulled down by immunoprecipitation against FLAG-tagged PCSK9 protein. Among 22 potential novel binding proteins identified, we found that the cellular inhibitor of apoptosis protein 1 (c-IAP1^10^) and the TNF receptor-associated factor 2 (TRAF2^11^) complex are regulated differently in different dominant PCSK9 mutations that occur naturally. Further immunoprecipitation analysis showed that c-IAP1 is a direct binding partner for PCSK9. One of the "gain-of-function" mutants, PCSK9-S127R, which has impaired autocatalytic activity, is defective in binding to c-IAP1. The other dominant mutation, PCSK9-D374Y^12^, which is 10-fold more potent in degrading the LDLR protein than wild-type PCSK9, can be significantly ubiquitinated by c-IAP1 in vitro. The ubiquitinated PCSK9-D374Y is unable to degrade LDLR, which is its main cause of hypercholesterolaemia in patients. These results indicate that there is a novel cholesterol uptake regulation pathway linking PCSK9/LDLR to the E3 ubiquitin ligase c-IAP1 in a TNF-[alpha] response pathway. This highlights the possibility of developing new treatments for human cardiovascular diseases through ubiquitin ligase-mediated ubiquitination of target proteins in cholesterol metabolism

Reasoning about Cardinal Directions between Extended Objects

Direction relations between extended spatial objects are important commonsense knowledge. Recently, Goyal and Egenhofer proposed a formal model, known as Cardinal Direction Calculus (CDC), for representing direction relations between connected plane regions. CDC is perhaps the most expressive qualitative calculus for directional information, and has attracted increasing interest from areas such as artificial intelligence, geographical information science, and image retrieval. Given a network of CDC constraints, the consistency problem is deciding if the network is realizable by connected regions in the real plane. This paper provides a cubic algorithm for checking consistency of basic CDC constraint networks, and proves that reasoning with CDC is in general an NP-Complete problem. For a consistent network of basic CDC constraints, our algorithm also returns a 'canonical' solution in cubic time. This cubic algorithm is also adapted to cope with cardinal directions between possibly disconnected regions, in which case currently the best algorithm is of time complexity O(n^5)