Project 3: myosin binding protein c



Molecular descriptions of muscle contraction typically focus on the proteins actin and myosin.  Descriptions of regulation focus on how the proteins troponin and tropomyosin affect myosin's interaction with actin.  But there are other "accessory" proteins in muscle.  Among others, these include the proteins titin and myosin binding protein c (MyBP-C, see Fig. 1).  There is an increasing recognition that these accessory proteins have a vital physiological role.


Although its function is unknown, MyBP-C is important, as mutations in its cardiac isoform are a leading cause of genetic heart disease. Recently, experiments have measured MyBP-C 's action in vitro.  These experiments suggest that it has a complex role, interacting with both contractile proteins, actin and myosin, and also with the regulatory protein tropomyosin.  Structural measurements further support this view.  As these experiments describe MyBP-C's action, there is a need for quantitative, mechanistic models to describe these data.




Figure 1: A schematic of MyBP-C interacting with the thick and thin filaments.  Inset shows the basic structure of MyBP-C, consisting of 8 immunogolobulin (Ig)-like domains and three fibronectin (FN)-like domains.  These are labeled C0-C10.  There are also disordered regions, e.g. the prolin-alanine-rich sequence (P/A) and the MyBP-C specific motif (M).  This latter region is an important regulatory site.





The goal of project 3 is to develop a quantitative, predictive model of MyBP-C's function.




Walcott, S., Docken, S. and Harris, S. P., Effects of cardiac myosin binding protein-C on actin motility are explained with a drag-activation-competition model.  Biophysical Journal (Letter) [In Press, PDF].


The cardiac isoform of myosin binding protein-C (cMyBP-C) has attracted recent attention because of its role in heart disease.  Despite this interest, its function is not well understood, partly because in some assays it can have multiple effects.  In the actin motility assay (Fig. 2A), for example, cMyBP-C can have a dual, biphasic activating/inhibitory effect. 


When the motility assay is performed at low calcium, thin filaments are largely immobile because tropomyosin inhibits myosin binding to actin.  Under these conditions, addition of cMyBP-C initiates motility.  However, as cMyBP-C is increased further, thin filament speed achieves a maximum, and eventually begins to decrease.  At high calcium, addition of cMyBP-C uniformly reduces thin filament speed (Fig. 2B).  This complex behavior is difficult to understand.



Figure 2: A drag-activation-competition model describes the effects of cMyBP-C on actin motility at both high and low calcium.  A. A schematic diagram of the motility assay.  Here, thin filaments (actin+troponin+tropomyosin) are observed moving over a glass surface that is densely coated with myosin.  cMyBP-C is subsequently added.  B.  Measurements of thin filament speed at high calcium (red) and low calcium (blue).  A model for cMyBP-C that includes drag, activation and competition fits the data.  Models lacking either drag or competition cannot fit the data. 



To elucidate the function of cMyBP-C, we developed a mathematical model of its interaction with the contractile proteins actin and myosin as well as the regulatory protein tropomyosin.  This model combines our previous model of myosin's interaction with actin (Project 1) and the local coupling between molecules due to the regulatory proteins (Project 2).  We used this model to fit published measurements of cMyBP-C in the actin motility assay.  These fits demonstrate that a drag-activation-competition mechanism is consistent with the data, while models lacking either drag or competition are not.  These three effects can arise simply from cMyBP-C binding to actin -- cMyBP-C binding to actin both displaces tropomyosin (leading to activation) and also precludes concurrent myosin binding (leading to competition).  Additionally, if cMyBP-C is attached to the glass surface, then binding to actin forms a transient link between the actin filament and the surface, generating a viscous drag that further slows motility.  Future use of this model may help distinguish mechanistic effects of cMyBP-C mutations that affect actin binding.




Steffen Docken

Samantha Harris