These clinically relevant arthroscopic landmarks may enhance single- and double-bundle ACL reconstructions through improved tunnel placement.

This study defines the radiographic locations of the femoral and tibial bundle attachment sites of the native ACL and a reliable and transferrable protocol for identifying these sites on radiographs in relation to surrounding landmarks and digitally projected reference lines. In addition, it was found that the femoral attachments of the AM and PL bundles were horizontally aligned at 115° of knee flexion and the PL bundle tibial attachment was located essentially at the center of the tibia.

This study provides a range of angles that can be used in double-bundle ACL reconstructions to restore normal knee stability without causing overconstraint.

We have detected directly the interactions of sarcolipin (SLN) and the sarcoplasmic reticulum Ca-ATPase (SERCA) by measuring fluorescence resonance energy transfer (FRET) between fusion proteins labeled with cyan fluorescent protein (donor) and yellow fluorescent protein (acceptor). SLN is a membrane protein that helps control contractility by regulating SERCA activity in fast-twitch and atrial muscle. Here we used FRET microscopy and spectroscopy with baculovirus expression in insect cells to provide direct evidence for: 1) oligomerization of SLN and 2) regulatory complex formation between SLN and the fast-twitch muscle Ca-ATPase (SERCA1a isoform). FRET experiments demonstrated that SLN monomers self-associate into dimers and higher order oligomers in the absence of SERCA, and that SLN monomers also bind to SERCA monomers in a 1:1 binary complex when the two proteins are coexpressed. FRET experiments further demonstrated that the binding affinity of SLN for itself is similar to that for SERCA. Mutating SLN residue isoleucine-17 to alanine (I17A) decreased the binding affinity of SLN self-association and converted higher order oligomers into monomers and dimers. The I17A mutation also decreased SLN binding affinity for SERCA but maintained 1:1 stoichiometry in the regulatory complex. Thus, isoleucine-17 plays dual roles in determining the distribution of SLN homooligomers and stabilizing the formation of SERCA-SLN heterodimers. FRET results for SLN self-association were supported by the effects of SLN expression in bacterial cells. We propose that SLN exists as multiple molecular species in muscle, including SERCA-free (monomer, dimer, oligomer) and SERCA-bound (heterodimer), with transmembrane zipper residues of SLN serving to stabilize oligomeric interactions. Sarcolipin (SLN)4 is a 3-kDa membrane protein that reversibly inhibits the activity of the calcium-transporting ATPase (SERCA) in sarcoplasmic reticulum (SR) of fast-twitch, slowtwitch, and atrial muscle (1, 2). SERCA is a 110-kDa membrane protein that relaxes muscle by pumping calcium out of the cytoplasm using energy derived from ATP hydrolysis (3). Transgenic mouse studies have demonstrated that knock-out of SLN enhances atrial contractility (4), cross-expression of SLN inhibits ventricular contractility (2, 5, 6), and ␤-adrenergic stimulation relieves SLN inhibition of contractility (2, 6, 7). Phosphorylation/dephosphorylation of SLN is responsible for SERCA regulation, controlling the rate and amount of calcium loading in SR, which in turn determines the rate of muscle relaxation and the force of subsequent contraction (2, 4 -7). SLN gene expression is up-regulated 2-15-fold in patients with skeletal muscle dysferlinopathy and Takotsubo cardiomyopathy, but down-regulated 2-3-fold in patients with heart failure, atrial fibrillation, and congenital heart defects, indicating that SLN acts as a causative or compensatory factor in human diseases of skeletal and cardiac muscle (8 -12).SLN comprises a single transmembrane (TM) helix, plus a small cytoplasm...