Final answer:
The yeast Ty virus-like particle lacks envelope proteins - surface protein and trans-membrane activity, crucial for infectivity in retroviruses, but not required for the Ty element's transposition in yeast.
Step-by-step explanation:
The yeast Ty virus-like particle (VLP) lacks envelope proteins, which are normally present in retroviruses to allow them to attach to and enter host cells. In retroviruses, the ENV gene encodes these envelope proteins. However, in the case of Ty elements in yeast, while they do encode the proteins necessary for reverse transcription and integration such as reverse transcriptase and integrase, they do not produce infectious viral particles because they lack the envelope proteins. Therefore, the correct answer to what is missing from the yeast Ty virus-like particle is option 2) Surface protein and trans-membrane activity.