Comparative studies on the functional roles of N‐ and C‐terminal regions of molluskan and vertebrate troponin‐I

Abstract

Vertebrate troponin regulates muscle contraction through alternative binding of the C‐terminal region of the inhibitory subunit, troponin‐I (TnI), to actin or troponin‐C (TnC) in a Ca2+‐dependent manner. To elucidate the molecular mechanisms of this regulation by molluskan troponin, we compared the functional properties of the recombinant fragments of Akazara scallop TnI and rabbit fast skeletal TnI. The C‐terminal fragment of Akazara scallop TnI (ATnI232−292), which contains the inhibitory region (residues 104–115 of rabbit TnI) and the regulatory TnC‐binding site (residues 116–131), bound actin‐tropomyosin and inhibited actomyosin‐tropomyosin Mg‐ATPase. However, it did not interact with TnC, even in the presence of Ca2+. These results indicated that the mechanism involved in the alternative binding of this region was not observed in molluskan troponin. On the other hand, ATnI130−252, which contains the structural TnC‐binding site (residues 1–30 of rabbit TnI) and the inhibitory region, bound strongly to both actin and TnC. Moreover, the ternary complex consisting of this fragment, troponin‐T, and TnC activated the ATPase in a Ca2+‐dependent manner almost as effectively as intact Akazara scallop troponin. Therefore, Akazara scallop troponin regulates the contraction through the activating mechanisms that involve the region spanning from the structural TnC‐binding site to the inhibitory region of TnI. Together with the observation that corresponding rabbit TnI‐fragment (RTnI1−116) shows similar activating effects, these findings suggest the importance of the TnI N‐terminal region not only for maintaining the structural integrity of troponin complex but also for Ca2+‐dependent activation.

Tissue-specific interactions of TNI isoforms with other TN subunits and tropomyosins in C. elegans: The role of the C- and N-terminal extensions

“…3a-b) are similar to those in a previous report that the nature of the TNI and TNC interaction is very different in different organisms and tissues [19]. The result of our overlay assays of TNC binding to TNI isoforms implies that the N-terminal regions of both the TNI-2 and TNI-4 isoforms interacted with pharyngeal TNC-2 ( Fig.…”

“…This was consistent to the case that molluskan Akazara scallop TNI 232-292 , which contained the actin/TNC-binding region, did not interact with TNC even in the presence of Ca 2+ [19]. It is of interest to study how molecular interactions among TN complex are developed during evolution.…”

“…Tanaka et al [19] reported that the ATNI 232-292 C-terminal fragment of Akazara scallop TNI inhibited actomyosin-tropomyosin Mg-ATPase activity. Al-Hillawi et al [20] also reported that the recombinant human cardiac TNI inhibited the acto-S1 Mg-ATPase activity.…”

Invertebrate muscles: Thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

“…However, it rapidly became clear that mollusc muscles contain tropomyosin and all three troponin components (Bailey and Rüegg, 1960;Konno, 1978;Goldberg and Lehman, 1978;Lehman et al, 1980;Lehman, 1983a;Shima et al, 1984;Takahashi and Morita, 1986;Ojima andNishita, 1986a,b, 1988a,;Ojima and Nishita, b) as well as a caldesmon-like protein (Bennett and Marston, 1990), that thin filament regulation was present in various molluscan muscles (Tsuchiya et al, 1978a;Lehman, 1981;Yazawa, 1985), and that the reason this regulatory system had been missed earlier is that high (but physiological for the organisms in question) Mg ++ levels are required to prevent tropomyosin and troponin dissociation during actomyosin purification (Lehman, 1983b). Later work has verified these results (Kambara et al, 1990;Ojima and Nishita, 1992b;Nishita et al, 1994Nishita et al, , 1997Ojima et al, 1994Ojima et al, , 1997Ojima et al, , 2000Ojima et al, , 2001Nishimura et al, 1997;Yumoto et al, 2003;Tanaka et al, 2005).…”

C. Elegans Model for Studying Tropomyosin and Troponin Regulations of Muscle Contraction and Animal Behavior

“…Recently we confirmed that the N-terminal part of troponin I interacts with troponin C of the worm (Amin and Kagawa, unpublished). This molecular nature is similar to the molluscan troponin I that interacts with the troponin C of the Akazara scallop (Tanaka et al, 2005). The importance of the N-terminal function of troponin I could be common throughout the invertebrates.…”