 Once formed , proteins only exist for a certain period of time and are then degraded and recycled by the cell 's machinery through the process of protein turnover . A protein 's lifespan is measured in terms of its half @-@ life and covers a wide range . They can exist for minutes or years with an average lifespan of 1 – 2 days in mammalian cells . <unk> and or <unk> proteins are degraded more rapidly either due to being targeted for destruction or due to being unstable . 
