Lipoprotein lipase (LPL) is responsible for the hydrolysis of triglycerides from circulating lipoproteins. Whereas most identified mutations in the LPL gene are deleterious, one mutation, LPL^S447X, causes a gain of function. This mutation truncates two amino acids from LPL's C-terminus. Carriers of LPL^S447X have decreased VLDL levels and increased HDL levels, a cardioprotective phenotype. LPL^S447X is used in Alipogene tiparvovec, the gene therapy product for individuals with familial LPL deficiency. It is unclear why LPL^S447X results in a serum lipid profile more favorable than that of LPL. In vitro reports vary as to whether LPL^S447X is more active than LPL. We report a comprehensive, biochemical comparison of purified LPL^S447X and LPL dimers. We found no difference in specific activity on synthetic and natural substrates. We also did not observe a difference in the K_i for ANGPTL4 inhibition of LPL^S447X relative to that of LPL. Finally, we analyzed LPL-mediated uptake of fluorescently labeled lipoprotein particles and found that LPL^S447X enhanced lipoprotein uptake to a greater degree than LPL did. An LPL structural model suggests that the LPL^S447X truncation exposes residues implicated in LPL binding to uptake receptors.

The signal recognition particle (SRP) directs ribosome-nascent chain complexes (RNCs) displaying signal sequences to protein translocation channels in the plasma membrane of prokaryotes and endoplasmic reticulum of eukaryotes. It was initially proposed that SRP binds the signal sequence when it emerges from an RNC and that successful binding becomes impaired as translation extends the nascent chain, moving the signal sequence away from SRP on the ribosomal surface. Later studies drew this simple model into question, proposing that SRP binding is unaffected by nascent chain length. Here, we reinvestigate this issue using two novel and independent fluorescence resonance energy transfer assays. We show that the arrival and dissociation rates of SRP binding to RNCs vary according to nascent chain length, resulting in the highest affinity shortly after a functional signal sequence emerges from the ribosome. Moreover, we show that SRP binds RNCs in multiple and interconverting conformations, and that conversely, RNCs exist in two conformations distinguished by SRP interaction kinetics.