prOGraMMIng PuZZleS & cOde ____
05AB1E, 18 bytes

Code:

áS.u5ôJC&gt;.bv'_y.;l


Uses the 05AB1E encoding. Try it online!

Explanation:

á                       # Remove non-letters from the input string.
 S                      # Split the result into individual characters.
  .u                    # Check if is uppercase for each character.
    5ôJ                 # Split into binary numbers of length 5.
       C                # Convert from binary to decimal.
        &gt;               # Add one.
         .b             # Map 1 → A, 2 → B, 3 → C, ..., 25 → Y, 26 → Z.
           v            # For each letter:
            '_y.;       #   Replace the first occurrence of '_' with the current letter.
                 l      #   Convert the string to lowercase.