In order to explore the effects of structural proteins on the quality changes of Longissimus dorsi muscle from Qinchuan cattle during storage, 4D-label free quantification (4D-LFQ) was used to analyze the proteomic changes of beef Longissimus dorsi muscle during different storage periods (0, 2, 4, 6 and 8 days). The changes in shear force, myofibrillar fragmentation index (MFI) and protein content were measured as well. The results showed that the shear force tended to increase and then decrease during the storage period of 8 days (P < 0.01), the increase being greater than the decrease. The MFI showed a significant upward trend with storage time (P < 0.01), increasing by up to 250.81% after 8 days of storage; however, the opposite trend was observed for the total soluble protein content (P < 0.05), decreasing by up to 34.60%. Structural proteins in muscle tissue were degraded with postmortem metabolic changes in muscle tissue, which possibly affected tenderness development and caused a significant decrease in the content of myofibrillar protein (P < 0.05). The content of myofibrillar protein decreased rapidly at first and then slowly, decreasing by 50.56% over the entire storage period. During the first four days, the abundance of four proteins (alpha-actin-1, myosin heavy chain 9, myosin regulatory light chain 2, and myosin regulatory light chain 12B) were changed by regulating the calcium ion-binding and cytoskeletal protein-binding pathways in the development of skeletal muscle tissue. During the 8 days of storage, the abundance of eight proteins (myosin regulatory light chain 2, myosin heavy chain 6, alpha-actin-1, actin, alpha cardiac muscle 1, myosin regulatory light chain 2, troponin Ⅰ 1, troponin Ⅰ 2, and myosin heavy chain 15) was changed by regulating the calcium ion-binding pathway in the development of muscle organ and striated muscle tissue. Moreover, the structural proteins were degraded as a result of the regulation of the physiological state of cells by myosin binding, calcium ion binding, cytoskeletal protein binding myofibrillar assembly, skeletal muscle tissue development, muscle organ development, and striated muscle tissue development, leading to an increase in the MFI, thus improving the tenderness.

In order to investigate the effect of differential expression of heat shock protein family A (HSP70) member 6 (HSPA6) on the meat quality of Qinchuan cattle during postmortem maturation, the Longissimus dorsi muscle of Qinchuan cattle was stored at 4 ℃ and evaluated for meat quality at 0, 2, 4, 6 and 8 days postmortem. 4D-Label free quantification (4D-LFQ) was used to analyze the expression of HSPA6 and related differential proteins. The results showed that the expression of HSPA6 decreased with increasing storage time, the contents of adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP) and reduced form of nicotinamide-adenine dinucleotide (NADH) decreased, the myofibril fragmentation index (MFI) increased, and the shear force and centrifugal loss increased and then decreased. Correlation analysis showed that HSPA6 expression was positively correlated with the contents of ATP and NADH and shear force (P < 0.05) and negatively correlated with MFI and centrifugal loss (P < 0.01), indicating that HSPA6 degradation was closely related to energy metabolism and water retention in beef during storage. Proteomics identified 24 significantly differentially expressed proteins (DEPs) associated with HSPA6 expression, which could cause proteasome-mediated ubiquitin-dependent proteolytic metabolism and cellular proteolysis through carbohydrate derivative binding and purine nucleoside triphosphate binding, in turn causing metabolic disorders or imbalances. These DEPs could also control protein degradation and cell apoptosis, in turn affecting structural changes in muscle and reducing tissue energy levels, and ultimately affecting the tenderness, MFI and water retention of Qinchuan beef through resistance to cellular structural protein degradation.